Difficult Swallowing in Cats with Hyperthyroidism

We have a 13-year old male cat named Balki with hyperthyroidism and moderate renal disease. He has a history of diabetes, but that has been in remission for the latter for several months.

Balki's main problem at the moment is trouble eating. My veterinarian just puts it down to the renal disease. However, it seems he wants to eat, but has difficulty swallowing. It has progressively gotten worse, to the point where he is now barely eating.

He also has had several episodes of reverse sneezing. When trying to eat, he extends his neck and gulps, then turns away from the food being offered. He also sometimes drools, and often spits out the food. He has most difficulty with dry food.

Can you give any advice on what might be causing this, and what tests he might need to find out, and how to help him to be able to eat?

My Response:

The typical hyperthyroid cat eats well (usually an increased appetite). These cats almost never develop anorexia, and difficulty swallowing (dysphagia) never occurs as a direct result of hyperthyroidism (1-4).

Most cats with moderate to severe renal disease will develop a decreased appetite as a result of their uremia, but they don't show signs of dysphagia (5). Most likely, your cat has disease either in the caudal pharygeal (throat) region or esophagus that is causing some obstruction to the food eaten (6-10). With the history of reverse sneezing, I'd say the problem is most likely in the caudal pharynx.

I'd recommend starting with a good oral examination (under sedation) to look for a lesion or mass in the pharygeal area. If nothing is seen, then endoscopy may be needed. Radiography or other imaging (e.g., CT scan) may also be required to help define the extent of your cat's disease (9-11).

Could thyroid tumors ever grow large enough to compress the esophagus and produce signs of dysphagia? That would be extremely unlikely, since even cats with large thyroid carcinomas almost always continue to eat well and don't have any problems swallowing.  However, I have had two cats in my career that had thyroid carcinoma which invaded the esophagus, leading to signs of esophagitis and esophageal obstruction. In any case, the chance of that being the problem in your cat would be about 1 in a million; again, that would best be diagnosed with endoscopy and CT imaging.

References:

1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
2. Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;199-203.
3. Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
4. Peterson ME. Top 10 signs of hyperthyroidism in cats. Animal Endocrine Clinic blog post. March 21, 2011.
5. DiBartola SP, Rutgers HC, Zack PM, et al. Clinicopathologic findings associated with chronic renal disease in cats: 74 cases (1973-1984). J Am Vet Med Assoc 1987;190:1196-1202. 
6. Watrous BJ. Clinical presentation and diagnosis of dysphagia. Vet Clin North Am Small Anim Pract 1983;13:437-459.  
7. Vos JH, van der Gaag I. Canine and feline oral-pharyngeal tumours. Zentralbl Veterinarmed A 1987;34:420-427.
8. Mattson A. Pharyngeal disorders. Vet Clin North Am Small Anim Pract 1994;24:825-854. 
9. Gengler W. Gagging. In: Ettinger SJ, Feldman EC, eds.The Textbook of Veterinary Internal Medicine. St Louis, Mo, USA: Saunders Elsevier; 2010:189–191.  
10. Jergens AE. Diseases of the esophagus. In: Ettinger SJ, Feldman EC, eds. The Textbook of Veterinary Internal Medicine. St Louis, Mo, USA: Saunders Elsevier; 2010:1487–1499. 
11.  Pollard RE. Imaging evaluation of dogs and cats with dysphagia. ISRN Vet Sci 2012;2012:238505. 

Success Rates for Radioiodine Treatment in Hyperthyroid Cats

The ideal goal of radioiodine (131-I) therapy in cats with hyperthyroidism is to restore euthyroidism with a single dose of radioiodine without producing hypothyroidism. Indeed, most hyperthyroid cats treated with 131-I are cured by a single dose (1-7).

Recommended monitoring after radioiodine
In general, the cats should be monitored at 2 to 4 weeks and again at 3 months after discharge from the radioiodine facility (4,7). At both of these recheck times, a complete physical examination as well as routine laboratory testing (e.g., CBC, serum chemistry panel, urinalysis) and serum thyroid hormone determinations (i.e, a total T4 concentration at minimum) are recommended.

If the serum T4 falls to subnormal or low-normal values (< 1.5 µg/dl) and iatrogenic hypothyroidism is suspected, a complete thyroid profile (total and free T4, T3, cTSH concentrations) is recommended to help rule out that diagnosis (4,8). I'll be discussing transient and permanent hypothyroidism more in my upcoming posts.

Cure rate after radioiodine
Serum thyroid hormone concentrations are normal within 2 to 4 weeks of radioiodine treatment in approximately 85% of cats and in 95% of cats by 3 months (5,6). Although cats appear to feel better within days after treatment, the owner should notice gradual clinical improvement and resolution of the signs of hyperthyroidism during this 3-month period.

Note that these percentages indicate remission of the hyperthyroid state but do not reflect the incidence of iatrogenic hypothyroidism, which has a reported incidence as high as 30% in some studies (9). Other studies, which employ serum TSH values to help identify early hypothyroidism, indicate that the rate of hypothyroidism can be much higher (up to 80%), depending on the method of dose determination. (10).

Persistent hyperthyroidism—Causes of treatment failure 
Approximately 5% of cats fail to respond completely and remain hyperthyroid after treatment with radioiodine. In studies from my clinic, most cats with persistent hyperthyroidism have large thyroid tumors, severe hyperthyroidism, and very high serum T4 concentrations (11). Other cats with mild-to-moderate hyperthyroidism may have a lower-than-expected thyroid 131-I uptake or show rapid turnover of the administered 131-I by the thyroid tumor (see below). In all these instances, treatment failure results because the radiation dose delivered to the tumor was inadequate to completely ablate the adenoma.

During the first 3 to 5 days of treatment with 131-I, we routinely measure daily neck radiation levels as an approximation of the cat’s thyroid iodine uptake value. This is useful in estimating the 131-I residence time in the thyroid tumor and can help determine the cause of treatment failure. In most hyperthyroid cats, the maximal thyroid radiation level is reached between 24 and 48 hours. 

Occasionally, a hyperthyroid cat shows an early peak thyroid uptake reading (i.e., sooner than 24 hours after dose administration), with lower thyroid uptake values at 24 and 48 hours. This early peak thyroid radioiodine uptake with rapid clearance is defined as “rapid iodine turnover” by the thyroid tumor (12). Such rapid turnover implies a short residence time for 131-I in the thyroid gland, which indicates that the administered 131-I dose may have a diminished radiation effect on the adenomas. In these cats, increased therapeutic 131-I dosages are needed to compensate for the decreased radiation effect to reduce the risk of persistent disease.

Persistent hyperthyroidism—Retreatment plan
If the hyperthyroid state persists in any cat for longer than 3 months after initial treatment, retreatment with radioiodine should be considered. In such cats, thyroid imaging (thyroid scintigraphy) and determination of thyroid uptake may help determine the cause of the initial treatment failure and ensure success with the second 131-I treatment.

In most of these cats, a 131-I dose (higher than that which was originally administered) will be needed to cure their hyperthyroid state. The prognosis remains good, however, and almost all can be cured with the second dose of radioiodine.

References:

1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
2. Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;199-203.
3. Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
4. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
5. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc 1995;207:1422-1428.  
6. Slater MR, Komkov A, Robinson LE, et al: Long-term follow up of hyperthyroid cats treated with iodine-131. Vet Radiol Ultrasound 1994;35:204-209.
7. Peterson ME. Radioiodine treatment of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:34-39. 
8. Peterson ME. Diagnostic testing for feline thyroid disease: Hypothyroidism. Compend Contin Educ Vet 2013:in press.
9. Nykamp SG, Dykes NL, Zarfoss MK, et al. Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002). J Am Vet Med Assoc 2005;226:1671-1675.
10. Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med 2010;24:1086-1092.
11. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats, in Conference Proceedings 29th Annual Veterinary Medical Forum (American College of Veterinary Internal Medicine) 2011;104-106.
12. Aktay R, Rezai K, Seabold JE, et al. Four- to twenty-four-hour uptake ratio: an index of rapid iodine-131 turnover in hyperthyroidism. J Nucl Med 1996; 37:1815-1819.

High-dose Radioiodine Treatment for Thyroid Carcinoma

Figure 1: Hyperthyroid cat with a huge thyroid tumor after 3 years of methimazole treatment. 
Notice the swollen neck, which turned out to be a massive thyroid carcinoma.

Thyroid carcinoma is a relatively rare cause of hyperthyroidism in cats (1-4). Our recent review of almost 2100 cats showed that less than 0.5% of cats with newly diagnosed hyperthyroidism will have thyroid cancer (5).

After months to years of methimazole treatment, it is not uncommon for the hyperthyroid cats' thyroid tumor(s) to continue to grow progressively larger (Fig. 1). Remember that all hyperthyroid cats have a thyroid tumor, which is responsible for the oversecretion of T4 and T3 (6,7).

In some of these cats, the thyroid tumor becomes very large (Fig. 1), and many of these will become difficult to regulate, even with high daily doses of oral or transdermal methimazole (8). Some cats eventually become completely refractory to methimazole or y/d, so alternative treatment modalities must be considered.

In addition to growing larger with time, the benign thyroid adenoma characteristic of early feline hyperthyroidism can also transform into malignant thyroid carcinoma in some cats (4,9,10). In our studies, the prevalence of thyroid carcinoma rises to over 20% in cats managed with methimazole longer than 4 years.

Again, methimazole or other antithyroid drug therapy (including Hill's y/d) does nothing to the thyroid tumor pathology and cannot stop the benign tumor from growing or transforming to carcinoma. Radioiodine therapy can be used to cure cats with such malignant thyroid tumors. Surgery can be attempted, but intrathoracic extension and metastasis is common, making it difficult to do a complete thyroidectomy these cats (Fig. 2).

Figure 2: Thyroid images of 6 hyperthyroid cats with thyroid carcinoma. 
Notice the large tumor volumes, with extension of disease beyond the limits of the thyroid capsule into the chest cavity in all cases.

High-dose Radioiodine Treatment for Thyroid Carcinoma

In cats with thyroid carcinoma, radioiodine offers the best chance for successful cure of the cancer because it concentrates in all hyperactive thyroid cells (i.e., carcinomatous tissue, as well as metastasis). Unlike cats with thyroid adenoma or adenomatous hyperplasia, the goal for cats with thyroid carcinoma is to totally ablate all thyroid tissue, ensuring complete destruction of all malignant thyroid tissue (Fig. 3).

Figure 3: Thyroid images of a hyperthyroid cat with thyroid carcinoma before and after high-dose radioiodine treatment.
Notice the complete ablation of all thyroid cancer tissue 2 months after treatment (right).

Because the tumor size and volume of thyroid carcinoma tissue is usually very large, extremely high doses of radioiodine (generally 10-fold higher than needed for most cats with benign thyroid disease) are almost always needed to destroy all of the malignant tissue. In most treatment centers that are licensed to treat with large doses of radioiodine, a high, fixed dose of 30 mCi (1110 mBq) is administered (4,11,12).

Longer periods of hospitalization are required with use of such high-dose radioiodine administration because of the prolonged radioiodine excretion. Because the goal is to ablate all thyroid tissue, this high dose almost always leads to iatrogenic hypothyroidism, necessitating daily L-thyroxine (L-T4) replacement therapy.

References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
3. Peterson ME: Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
4. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
5. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; 26:754.
6. Peterson ME. Do all cats have a thyroid tumor? Is it thyroid cancer? Insights into Veterinary Endocrinology. May 7, 2011.
7. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Vet Clinics  North Am Small Anim Pract 1994;24:541-565
8. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
9. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Proceedings of European College of Veterinary Internal Medicine; 2012.
10. Peterson ME. Thyroid tumors grow progressively larger in most hyperthyroid cats treated with methimazole. Insights into Veterinary Endocrinology. August 26,2012.
11. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
12. Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981-1986). J Am Vet Med Assoc 1988;193:359-364. 

Treating Dogs with Hyperthyroidism and Thyroid Carcinoma

I have a 12 year-old, spayed, female, Chocolate Lab named "Coco" who was diagnosed with hyperthyroidism. Coco has lost 2.5 pounds of weight over the last 6 months but otherwise she was asymptomatic.

Coco's routine laboratory workup was unremarkable but her serum thyroid hormone panel was diagnostic for hyperthyroidism.  The panel showed high serum concentrations of T4 (4.3 μg/dl; reference range, 1-4 μg/dl), T3 (209 ng/dl; reference range, 50-150 ng/dl) and free T4 (52 pmol/L; reference range, 10-50 pmol/L), and low serum concentrations of TSH (<0.03 ng/ml; reference range, 0-0.6 ng/ml).

Chest radiographs were performed but no tumor metastasis was detected. My veterinarian palpated a small (grape-sized), freely movable right thyroid mass on Coco and performed a unilateral thyroidectomy last week to remove the tumor. The pathology report states the following:

• The tumor was completely surrounded by a fibrous capsule
• No capsular or vascular invasion
• A few mitotic figures (7 mitotic figures seen with 10 high-powered fields)
• Diagnosis ”thyroid follicular tumor, probably adenoma but carcinoma cannot be excluded” 

Do you have any information that you can share (or guide me to) regarding thyroid tumors and hyperthyroidism in dogs? I can find a lot of information about cats (you seem to be THE expert in feline hyperthyroidism), but I can not find very much information about treatment of hyperthyroid dogs. I'd love to come and see you, but I live in Western Pennsylvania and it would be a very long drive.

Other than this, my Coco has always been in excellent condition/health.

My Response:

Most dogs with naturally occurring hyperthyroidism will have a thyroid tumor, and most of these overactive thyroid tumors will be malignant (thyroid carcinomas). Very few dogs with hyperthyroidism have benign thyroid tumors, but it looks like your dog may be one of the lucky ones.  Based on the small tumor size and noninvasive nature, the thyroid tumor is likely to be either benign in the early stages of malignancy. The fact that the chest x-ray did not detect any cancer metastasis is a good sign.

Additional treatments that could be considered include the use of radioactive iodine (I-131), chemotherapy, and/or local radiotherapy of the cervical tumor bed. Based on the fact that she was hyperthyroid, radioiodine would be my first choice of treatment since any residue tumor tissue should "take up" the injected I-131 very well. However, radioiodine facilities for dogs are limited, and, to my knowledge, no one in the State of Pennsylvania offers radioiodine treatments for dogs; the closest place that I know of is Michigan State University.

In your dog, however, further treatment may not be necessary. By surgically removing the thyroid tumor, it's very possible that Coco has been cured. I'd certainly follow-up with another serum thyroid hormone panel in 2-4 weeks to ensure that her values have all returned to normal range.  Additional thyroid testing should be done quarterly. If hyperthyroidism persists or recurs, that likely indicates local recurrence of the thyroid tumor tissue or tumor metastasis.

Likewise, I would continue to monitor Coco's chest x-rays at least quarterly, again looking for tumor metastasis. If pulmonary nodules are ever detected, that could indicate thyroid metastasis and additional therapy would certainly be needed. Hopefully, Coco will be one of the lucky dogs, and her thyroid tumor has been cured.

Suggested Reading:

1. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, 2006;327-342.
2. Peterson ME: Hyperthyroidism and thyroid tumor in dogs. In: Melian C, Perez Alenza MD, Peterson ME, Diaz M, Kooistra H (eds): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008;113-125.
3. Mooney CT. Canine hyperthyroidism In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;86-91.

My Related Posts on Hyperthyroidism in Dogs:

• Thyroid Tumors and Hyperthyroidism in Dogs  
• Canine Hyperthyroidism and the Heart 
• Dietary Hyperthyroidism in Dogs  

January is National Thyroid Awareness Month

"If your thyroid isn't working properly, neither are you."

January is Thyroid Awareness Month, which is sponsored by the American Association of Clinical Endocrinologists (AACE).  We must remember that thyroid disease commonly affects people, as well as our cats and dogs. The AACE also estimates that approximately 30 million Americans may be affected by thyroid disorders (hypothyroidism, hyperthyroidism, and thyroid cancer) — with half of these cases currently undiagnosed.

In humans, the thyroid gland is a small, butterfly-shaped gland found immediately below the Adam’s apple. This gland produces hormones that influence every organ, tissue and cell in your body. If thyroid disease is left untreated, there are serious consequences including elevated cholesterol levels, heart disease, infertility, muscle weakness, and osteoporosis.

Testing for thyroid disease is easy, although not routinely included in an annual physical blood work. However, the diagnosis of thyroid disease can sometimes be challenging. Patients often present with vague, general clinical manifestations that may not be obvious to either the doctor or patient. Understanding the facts about thyroid disease and its symptoms is the best defense in diagnosing and treating thyroid disease.

What Are The Symptoms Of Thyroid Disease In Humans?

The following are some of the symptoms of various thyroid conditions and diseases.

Hypothyroidism
The symptoms of hypothyroidism (i.e., an underactive thyroid) tend to mirror the slowing down of physical processes that result from insufficient thyroid hormone. Common symptoms include fatigue, weight gain, constipation, fuzzy thinking, low blood pressure, fluid retention, depression, body pain, slow reflexes, and much more.

Hyperthyroidism
The symptoms of hyperthyroidism (i.e., an overactive thyroid) tend to reflect the rapid metabolism that results from an oversupply of thyroid hormone. Common symptoms include anxiety, insomnia, rapid weight loss, diarrhea, high heart rate, high blood pressure, eye sensitivity or bulging, and vision disturbances.

Thyroid Nodules or Goiter
Symptoms of goiter — an enlarged thyroid gland— include a swollen, tender or tight feeling in the neck or throat, hoarseness or coughing, and difficulty swallowing or breathing. Sometimes, the goiter is visible to yourself or others.

Some thyroid nodules cause no symptoms, while others may cause difficulty swallowing, a feeling of fullness, pain or pressure in the neck, a hoarse voice, or neck tenderness. Some nodules trigger hyperthyroid-like symptoms such as palpitations, insomnia, weight loss, anxiety, and tremors. Nodules can also trigger hypothyroidism, and symptoms might include weight gain, fatigue, and depression.

Thyroid Cancer
Although many patients are asymptomatic at first, possible symptoms of thyroid cancer include a lump in the neck, voice changes, difficulty breathing or swallowing, or lymph node swelling.

Thyroiditis
Symptoms of thyroiditis typically include pain and tenderness in the thyroid area, neck and throat, difficulty sleeping. Thyroiditis may also trigger traditional hypothyroid or hyperthyroid symptoms. 

Where To Get More Information?

For more information on thyroid disease and/or to find a medical expert in thyroid conditions, please visit the American Association of Clinical Endocrinologists website. The AACE’s Thyroid Awareness website also features articles, videos and FAQ on thyroid conditions.

For a detailed description of various thyroid disorders in human patients, including hypothyroidism and Hashimoto’s thyroiditis, hyperthyroidism and Graves’ disease, multinodular goiter, thyroid nodules and thyroid cancer, I'd strongly recommend that you visit the Thyroid Disease Manager Website.

Thyroid Disease Manager offers an up-to-date analysis of all aspects of human thyroid disease and thyroid physiology. It provides physicians, researchers, and patients from around the world with an authoritative, current, complete, objective, free, and down-loadable source on the thyroid and its diseases.

Useful Links:

• American Association of Clinical Endocrinologists (AACE) website — https://www.aace.com/
• AACE's Thyroid Awareness website — www.thyroidawareness.com
• Thyroid Disease Manager website —www.thyroidmanager.org

Thyroid Tumors Grow Progressively Larger in Most Hyperthyroid Cats Treated with Methimazole

As I discussed in my last post (Managing Hyperthyroid Cats That Become Unresponsive to Methimazole), methimazole blocks thyroid hormone secretion from a hyperthyroid cat's thyroid tumor (usually a benign tumor). However, in cats treated with methimazole, it is quite common for hyperthyroid cats on methimazole treatment to need higher dosages of methimazole over time, as the thyroid adenoma continues to grow larger and increases its secretion of thyroid hormone (1-3).

No one has yet studied the long-term effects of nutritional management with a low-iodine diet (i.e., Hill's y/d) on thyroid tumor growth in these hyperthyroid cats. However, the same progressive growth of the thyroid tumor would be expected on a low-iodine diet, since the thyroid tumor remains and is free to continue to grow progressively larger with time.

Figure 1: Hyperthyroid cat who has developed a very large thyroid tumor after 3 years of methimazole treatment. Notice the swollen neck, which turned out to be a massive thyroid carcinoma.

After months to years of methimazole treatment, many of these cats will develop a very large, palpable thyroid tumor (Figure 1) and will become difficult to regulate, even with high daily doses of oral or transdermal methimazole (3-5). Some cats eventually become completely refractory to methimazole, so alternative treatment modalities must be considered.

With enough time and as the disease progresses, the benign thyroid adenoma characteristic of early feline hyperthyroidism can also transform into malignant thyroid carcinoma in some cats (5,6). Again, methimazole or other antithyroid drug therapy (including Hill's y/d) does nothing to the thyroid tumor pathology and cannot stop this from happening.

Radioiodine therapy can be used to successfully treat cats with all sizes of benign and malignant thyroid tumors, but is best used on cats with small benign tumors. Cats with small thyroid tumors are easier to cure with lower radioiodine doses, resulting in shorter hospitalization times.

Video Animation: Thyroid Growth on Methimazole

The animation below describes the continued growth of the initially benign thyroid tumors causing hyperthyroidism in cats during their medical management with methimazole (Tapazole).

This video was done by my colleague and friend, Dr. Michael Broome, Director of Advanced Veterinary Medical Imaging in Orange Country, California. We have been long-term collaborators on clinical research issues pertaining to cats with hyperthyroidism, and we continue to work closely together on this common disorder (4,5).




References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
3. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
4. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; 26:754.
5. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Proceedings of European College of Veterinary Internal Medicine; 2012.
6. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.

Managing Hyperthyroid Cats that Become Unresponsive to Methimazole

Antithyroid drugs (methimazole or carbimazole) are commonly used as the primary means of long-term therapy for cats with hyperthyroidism. During chronic treatment (i.e., over a period of many weeks to years), many cats will develop a "resistance" to this medication. These cats will need progressively higher daily dosages of methimazole (or carbimazole) in order to keep serum thyroid hormone values within the normal, reference range limits (1-3).

The purpose of this post is to address the following questions:

• What's the highest methimazole dose that can be given?
• What causes this methimazole resistance during long-term antithyroid drug treatment?
• Could this be caused by thyroid cancer?
• What other treatments should be considered?

What's the Highest Methimazole Dose That Can Be Given to Cats?
There is no maximum dose of methimazole for cats.  However, when methimazole doses approach 10 to 15 mg twice daily (20-30 mg/day) and the serum T4 remains high, it is advisable to consider other treatment options (see below).

However, several issues should be first considered.

1. First, ensure that the tablets are consistently being put into the cat's mouth and that the cat is swallowing the pills. Many cats will learn to hold the pill in their mouth and spit it out later when the owner is not watching.  If the medication is given in food, it is common for cats to "eat around" the pill and not swallow it. If oral medication becomes an issue in treatment,  transdermal methimazole can be considered (4,5). 
2. Secondly, the problem may be the use of generic formulations of methimazole, which are not always as bioavailable or effective as brand-name products (Felimazole; Tapazole). A change to a brand-name product can sometimes be helpful.
3. Third, gastrointestinal problems (e.g., inflammatory bowl disease) could be affecting methimazole absorption. If the cat has chronic vomiting or diarrhea, a workup for primary intestinal disease might be required.
4. Finally, we must consider the possibility that a thyroid cancer (carcinoma) has developed, which generally is less responsive to the methimazole than benign thyroid tumors (see below).

Other than the last issue, most of these problems would have been addressed early on in the course of medical management. So these issues are typically not the cause of the methimazole resistance that we see in hyperthyroid cats treated chronically with antithyroid drugs.

Why is the Dose of Methimazole Increasing?
After methimazole (or carbimazole) is administered, the thyroid gland takes up and concentrates the antithyroid from the circulation. Once within the thyroid, methimazole works by inhibiting the production of T4 and T3 from the hyperthyroid cat's thyroid tumor (4,5).

It is imperative to understand that methimazole treatment blocks T4 and T3 production from the hyperthyroid cat's thyroid tumor but does not cure the disease. In cats treated with methimazole, the underlying cause of the hyperthyroidism (a benign thyroid tumor called an adenoma) remains intact. It is, therefore, quite common for hyperthyroid cats on methimazole treatment to need higher dosages of methimazole over time, as the thyroid adenoma continues to grow larger or increases its secretion of thyroid hormone (1-3,6).

Figure 1: Hyperthyroid cat that has developed a large thyroid tumor
after 18 months of methimazole treatment

After months to years of treatment, many of these cats will develop a very large, palpable thyroid tumor (Figure 1) and will become difficult to regulate, even with high daily doses of oral or transdermal methimazole. Some cats eventually become completely refractory to methimazole, so alternative treatment modalities must be considered.

Could This Be Thyroid Cancer?
With enough time and as the disease progresses, the benign thyroid adenoma characteristic of early feline hyperthyroidism can also transform into malignant thyroid carcinoma in some cats (7,8). Again, methimazole or other antithyroid drug therapy does nothing to the thyroid tumor pathology and cannot stop this from happening.

Figure 2: The thyroid scan on the left is from a cat that has become unresponsive to methimazole after 3 years of treatment.  This cat has massive thyroid tumors located both in the neck area but also extending well into the chest cavity (compare to normal thyroid scan on right). This is considered diagnostic for thyroid carcinoma.

As someone who does thyroid imaging and radioiodine treatment, I commonly see cats that have been treated for years with antithyroid drugs and now have very large to huge thyroid tumors. In about 10% of these cats with severe, long-standing hyperthyroidism, the diagnosis is thyroid carcinoma with extension of tumor or metastasis into the chest (Figure 2). It's quite likely that transformation of the tumor from adenoma to carcinoma has occurred in these cats. This may be the natural evolution of the disease; however, antithyroid drugs have been shown to be carcinogenic in rodents, it is possible that the drug treatment is playing a role.

Do All Hyperthyroid Cats Exhibit Progressive Growth of Their Thyroid Tumor(s)?
Fortunately, all cats do not show accelerated tumor growth on methimazole. In a few cats, the thyroid tumor grows very slowly or not at all (7). Most cats, however, definitely have an increase in thyroid size over time. And some cats show a rapid increase in size, just within a few months.

Unfortunately, in a newly diagnosed hyperthyroid cat, we have no way to determine how fast the thyroid tumor is going to grow or its potential for malignant transformation.

What Other Treatments Should Be Considered in Cats Unresponsive To Methimazole?
In cats that develop large thyroid tumors and become resistant to the effects of methimazole, we have 3 options for treatment: radioiodine, surgical thyroidectomy, or nutritional management with a low-iodine diet (Hill's y/d).  Of these 3 options, radioiodine is the treatment option of choice.

Surgery has a major advantage over methimazole and nutritional therapy because the large, potentially malignant thyroid masses are removed and the cat is potentially cured (9). However, complications of surgery are intensified in cats with large thyroid masses. First of all, these larger tumors are very vascular, may be invasive, and can bleed during the surgical procedure. This can make it difficult to identify the parathyroid glands, which must be preserved to prevent a low serum calcium from developing postoperatively. In addition, many of these cats have thyroid tumors that now extend into the thoracic cavity or have metastasized. Therefore, complete surgical removal of all thyroid tumor tissue may not be possible.

In most cats with severe hyperthyroidism and large thyroid masses, the use of nutritional management is not successful in normalizing thyroid hormone concentrations, similar to the methimazole. Even if this form of therapy did successfully lower serum T4 values to normal, the large and potentially malignant thyroid tumors remain, enabling them to continue to grow and possibly metastasize.

Although use of radioiodine is generally successful in treating cats that have been unresponsive to methimazole, these cats can be much more difficult to cure with a single dose of radioiodine than are cats with smaller thyroid tumors or recently diagnosed hyperthyroidism (1-3). Cats with huge benign or malignant tumors require much larger doses of radioiodine to ablate (10 to 30 mCi) the thyroid tumor(s) than do the typical recently diagnosed cats with mild to moderately hyperthyroidism (2 to 6 mCi).  Because these cats have been treated for months to years, they also tend to be older and have many more complications, mainly because of the concurrent diseases (e.g., renal disease) that are common in older cats.

References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
3. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
4. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22–28.
5. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North American Small Animal Practice 2007;37:775-788.
6. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; 26:754.
7. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Proceedings of European College of Veterinary Internal Medicine; 2012.
8. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
9. Radlinsky MG. Thyroid surgery in dogs and cats. Veterinary Clinics of North American Small Animal Practice 2007;37:789-798.

How Many Ways Can the Thyroid Malfunction?

Thyroid disease (i.e., hypothyroidism, hyperthyroidism, and thyroid tumors) is common in dogs and cats. Dr. Ann Hohenhaus, an oncologist and former colleague of mine at the Animal Medical Center in New York City, wrote the following blog post on WebMD about 2 dogs and a cat with thyroid disease that I thought was worth sharing.

The first case concerns a dog with thyroid cancer; the second case a hyperthyroid cat who previously had intestinal lymphoma (a cancer); and the third case of a dog with hypothyroidism.

How Many Ways Can the Thyroid Malfunction?
By Ann Hohenhaus, DVM

The thyroid gland sits in the neck of dogs and cats, just below the voice box, and controls metabolic functions. Most of the time, a routine physical examination cannot detect the organ if it is normal.

Last week, my patient list ran the gamut of thyroid dysfunction. Here is a sampling:

A Tail of Two Thyroids
Some days, strange coincidences happen in the waiting room. Today it was two dogs, both with thyroid cancer. Although measuring 15 centimeters in length, Beckey’s thyroid tumor had been surgically removed. The biopsy showed her tumor trying to escape into the lymph vessels and she was waiting her turn for chemotherapy, administered to halt the spread. Her treatment involves intravenous administration of two different chemotherapy agents and Beckey so far has sailed through the treatment with flying colors.

As Beckey was leaving the waiting room, Henry entered. A CT scan showed his thyroid tumor had already spread to the lymph nodes in his neck, precluding surgical removal. He was in for a check-up following completion of four radiation therapy treatments. Careful measurement of his tumor with calipers showed no increase in tumor size. The radiation treatment arrested tumor growth but had given him a sore esophagus. I had warned the owners about this type of side effect before we started treatment and told them to expect it to start resolving about two weeks after he completed his treatment. Henry did not disappoint us. Through telephone triage, we had already rearranged his medications to make his throat less painful. Henry spends summer in the country but in the fall he will come back to The AMC for measurement of the tumor and a chest x-ray.

Old Patient, New Problem
Otra’s family was worried. This cute kitty had completed chemotherapy for intestinal lymphoma about a year ago, but suddenly her weight plummeted. I could see from the look on their faces they were sure the cancer was back. Auscultation of Otra’s heart discovered a very elevated heart rate, prompting a test of her thyroid levels. Overactive thyroid glands ramp up the cat’s metabolism and they lose weight despite eating well, have a high heart rate, and are very peppy. An abdominal ultrasound showed no evidence the lymphoma had recurred and blood tests showed the thyroid was overactive. I sent thyroid-suppressing medications home with the relieved family and planned to reassess the thyroid hormone levels in two weeks.

Porterhouse to Pork Chop
Every time I saw Mango to follow up on a skin tumor that had been completely removed via surgery, she had gained another pound. This 60-pound Portuguese Water Dog should have weighed 50 pounds. The owners took her swimming, fed her diet food from feeding toys, and still she gained two more pounds. During an evaluation for a urinary tract infection, we noted her thyroid hormone levels were borderline low. When we retested the levels three months later, we confirmed diagnosis of hypothyroidism. Low thyroid function, the opposite of Otra’s problem, can cause weight gain. Since she started treatment with thyroid supplementation, Mango has lost nearly 6 pounds and gone from a 20-ounce porterhouse to a 4-ounce pork chop over the past few months!

There you have it, thyroid malfunction runs the gamut of disease: overactive, underactive, and two different tumors, all in one tiny organ.

My Related Posts:

• Hypothyroidism: One of the Top 10 Diagnosed Diseases of Dogs 
• How is Canine Hypothyroidism Diagnosed? 
• How Do We Confirm a Diagnosis of Hypothyroidism in Dogs? 
• How Do We Treat Dogs With Hypothyroidism? 
• Hyperthyroid in Cats: Table of Contents 
• Treatment Options and Considerations for Hyperthyroid Cats 
• More Hyperthyroid Posts, etc 

Thyroid Tumors and Hyperthyroidism in Dogs

Large neck mass in a dog with
thyroid cancer (carcinoma)

In dogs, thyroid tumors are common, representing approximately 1 to 4% of all canine neoplasms. Most thyroid tumors in the dog are non-functional and do not lead to hyperthyroidism — completely different from the situation in cats, where most cats with thyroid tumors are hyperthyroid (for more information, see links to my related posts, below). 

As opposed to the relatively small, non-invasive and benign thyroid tumors associated with hyperthyroidism in cats, most clinically detected thyroid tumors in dogs are large, invasive tumors associated with thyroid cancer (carcinomas).

Nature of Thyroid Cancer in Dogs
In dogs with thyroid carcinoma, both local invasion of tumor into adjacent structures (e.g., esophagus, trachea, cervical musculature, nerves, or thyroid vessels) and distant metastasis (e.g., lung, liver, or regional lymph nodes) are common.

Approximately 50% of dogs with thyroid carcinoma have documented metastasis at time of diagnosis. During the natural course of disease, 65% to 90% of dogs with untreated disease will develop metastasis to a wide variety of tissue.

Thyroid Tumors and Hyper- and Hypothyroidism
In most dogs with thyroid tumors, their thyroid function remains normal. Up to a third of dogs with thyroid cancer will develop hypothyroidism from destruction of normal thyroid tissue.

Only 10% of thyroid carcinomas in dogs are overactive, producing clinical signs of hyperthyroidism. Although thyroid adenomas producing hyperthyroidism have been described, the incidence of benign thyroid tumors causing hyperthyroidism in dogs is very low. In dogs, a thyroid tumor causing hyperthyroidism should always be presumed to be a carcinoma until proven otherwise. Again, this is in stark contrast to the situation in hyperthyroid cats, in which thyroid carcinoma is rare (less than 5% of in cats).

Clinical Features 
Most dogs with thyroid tumors are of middle- to old-age (greater than 5 years; average age, 10 years). There is no sex predilection. Breeds reported to be at increased risk of developing thyroid tumors include boxers, beagles, Golden Retrievers, and Labrador Retrievers.

Many dogs with thyroid tumors are presented because the owner has noticed an enlargement of the neck. In more than 75% of dogs diagnosed in one survey, either the cervical swelling was the only reason for seeking veterinary care or the thyroid mass was detected by the veterinarian during an examination for another problem.

Because of the large tumor volume and high incidence of both local invasion and distant metastasis, clinical signs such as trouble breathing (dyspnea), cough, hoarseness, alternation in bark, vomiting, poor appetite, and weight loss may be reported, especially in dogs with nonfunctional thyroid tumors. A surprising number of owners, however, fail to report any clinical signs in their dogs.

Of all thyroid tumors in dogs, approximately 10% will become hyperthyroid, and in these dogs, the hyperthyroid state is usually the major reason for examination. Increased thirst and urination are usually the earliest and most predominant signs associated with hyperthyroidism in dogs. Weight loss, despite an increase in appetite, is also common.

Diagnosis
Thyroid imaging or scanning (thyroid scintigraphy) is useful in the evaluation of dogs with thyroid tumors because the procedure can help locate where the thyroid tissue is located (both normal tissue, as well as benign and malignant tumors).  Thyroid imaging can also be performed to help determine the extent of thyroid invasion or metastasis.  

Definitive diagnosis of thyroid carcinoma is best made by either incisional biopsy or surgical excision of the thyroid mass, followed by histological examination of the thyroid tissue. Despite the fact that most thyroid tumors detected in clinical practice are easily palpable, they are not simple to biopsy. Because of the vascular and invasive nature of canine thyroid carcinomas, hemorrhage is a common complication.

Treatment
Treatment of thyroid neoplasia in dogs is dictated by the size of the primary tumor, extent of local tissue invasion, presence of detectable metastasis, presence of hyper- or hypothyroidism, and available treatment options.

Because most clinically detected thyroid tumors in the dog are malignant, treatment is rarely curative. Nevertheless, one should generally advise some form of treatment because palliative relief and increased lifespan can usually be achieved in most dogs with thyroid carcinoma.

Surgical removal of a small thyroid tumor in a dog

Surgery, chemotherapy, cobalt irradiation, and use of radioactive iodine therapy, alone or in combination, may be indicated depending on the individual dog. Medical control of the hyperthyroid state can be achieved by the daily administration of an antithyroid drug such as methimazole or carbimazole (5 to 15 mg/dog, twice, daily), but such medical treatment will not do anything to prevent tumor growth or metastasis.

Suggested Reading:

1. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, 2006;327-342.
2. Peterson ME: Hyperthyroidism and thyroid tumor in dogs. In: Melian C, Perez Alenza MD, Peterson ME, Diaz M, Kooistra H (eds): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008;113-125.
3. Mooney CT. Canine hyperthyroidism In: Mooney CT,Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;86-91.

My Related Posts on Thyroid Disease in Dogs and Cats:

• Hypothyroidism: One of the Top 10 Diagnosed Diseases of Dogs 
• How is Canine Hypothyroidism Diagnosed? 
• How Do We Confirm a Diagnosis of Hypothyroidism in Dogs? 
• How Do We Treat Dogs With Hypothyroidism? 
• Hyperthyroid in Cats: Table of Contents 
• Treatment Options and Considerations for Hyperthyroid Cats 
• More Hyperthyroid Posts, etc 

Is Thyroid Scanning Useful in Cats with Hyperthyroidism?

Thyroid scintigraphy (thyroid scanning by nuclear imaging) is a powerful imaging technique for visualizing thyroid tumors.  Only a few specialized facilities (such as the Animal Endocrine Clinic) have the equipment and special licensing required to perform scintigraphy.  In this week’s blog, I will be discussing how thyroid scintigraphy works, and its four main uses: diagnosing hyperthyroidism, locating impalpable tumors (tumors that cannot be felt on exam), diagnosing thyroid carcinomas, and calculating a radioiodine dose.

How thyroid scintigraphy works
To perform thyroid scintigraphy, the specialist injects the cat with a small dose of a radioactive tracer subcutaneously.  Over the next hour, the cat’s salivary glands and thyroid glands take up the tracer.  The radioactive tracer, now in the salivary and thyroid glands, then emits gamma rays (a high energy electromagnetic wave, a bit stronger than an X-ray), which are detected by a gamma camera to form an image.

Thyroid scintigraphy for diagnosing hyperthyroidism
Sometimes, even an endocrinologist will have trouble telling whether a cat is hyperthyroid.  Its T4 values may be on the borderline of abnormal, and the tumor may be difficult to palpate.  Thyroid scintigraphy is the “gold standard” for determining whether the cat is hyperthyroid.

In a normal cat, the thyroid gland will take up about as much of the tracer as the salivary glands do (see Figure 1).  On the scan, we expect the thyroid and salivary glands to be equally bright (a 1:1 brightness ratio).

Figure 1: Thyroid scan of a normal cat.  

Note that the salivary and thyroid glands are equally bright.

However, in a hyperthyroid cat, the adenoma cells take up far more of the tracer than the salivary glands do (see Figures 2 & 3).  Therefore, the adenomas show up far more brightly than the salivary glands.  This definitively diagnoses hyperthyroidism.  In order to diagnose hyperthyroidism in borderline cases, the specialist must use the software to carefully measure the densities of the thyroid and salivary glands.

Figure 2: Thyroid scan of a cat with unilateral right-sided thyroid adenoma.  

Note the brightness of the adenoma compared to the brightness of the salivary glands.

Figure 3: Thyroid scan of a cat with bilateral (two-sided) thyroid adenomas.  

Note the brightness of the adenomas compared to the brightness of the salivary glands.

Locating impalpable tumors
Impalpable tumors are fairly common in hyperthyroid cats; they affect about one-third of hyperthyroid cats.  These impalpable tumors come in two forms:  ectopic (not in the normal thyroid region) and large tumors that gravity pulls into the chest cavity.

Ectopic thyroid tissue is an embryological phenomenon that we see fairly often.  As the cells that develop into the thyroid gland traverse the embryo, they can leave traces of tissue anywhere from the base of the tongue to the base of the heart.   These traces can develop into functional thyroid tissue.  If these tissues develop a tumor, it will be impalpable, especially if the ectopic tumor is located on the tongue or in the chest cavity.  A thyroid scan can quickly and easily locate these tumors (see Figure 4).

Figure 4: Thyroid scan of a cat with an ectopic thyroid adenoma in the chest cavity. 

The veterinarian will not be able to palpate this tumor because of its location.

Like ectopic tumors, large tumors that descend into the chest cavity are impalpable.  We often see these tumors in cats that have been hyperthyroid for many months.  As the tumor grows progressively larger, gravity pulls progressively harder.  This causes the tumor to descend into the chest cavity.  Again, a thyroid scan swiftly detects these kinds of thyroid tumors (see Figure 5).

Figure 5: Thyroid scan of a cat with bilateral thyroid tumors. The left thyroid adenoma is small but should be palpable. The right thyroid tumor is very large and gravity has pulled it down into the thoracic cavity. The veterinarian would not be able to feel this tumor either.

Diagnosing thyroid carcinomas
As I discussed in a previous blog, some hyperthyroid cats have malignant thyroid carcinomas.  Neither palpation nor blood tests can determine whether a hyperthyroid cat has an adenoma or carcinoma.  Scintigraphy is extremely helpful in diagnosing invasive carcinomas.  On a scan, a carcinoma will appear as a large, irregularly shaped tumor extending beyond the limits of the normal thyroid region (see Figure 6).  We can also see whether the tumor has metastasized.

Figure 6: Thyroid scan of a cat with a thyroid carcinoma.  

Note that the tumor is irregularly shaped and is spreading into the chest cavity.

Diagnosing thyroid carcinomas is essential.  Without thyroid scintigraphy, a cat with a thyroid carcinoma would not receive the ultra-high dose of radioiodine necessary to destroy the cancer.

Calculating the radioiodine dose
In addition to diagnosing carcinomas (see above), thyroid scintigraphy helps us to make sure hyperthyroid cats receive the appropriate dose of radioiodine. Scintigraphy can precisely calculate the size of the tumor, even if it is impalpable.  With this information, we can calculate the dose of radioiodine for the cat, tailored specifically to that cat’s tumors.

Examples of recent thyroid scans
To see several examples of recent thyroid scans performed at our Bedford Hills facility, along with their accompanying interpretations, please visit our Nuclear Imaging Facebook page.