Monday, June 25, 2012

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.

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 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.


Barbara Goll said...

If the dosage recommended is 5 to 15 mg twice a day for dogs, what criteria are used to determine the dosage? Is it dependent upon the weight of the dog, the severity of the symptoms, or the T4 level?

Dr. Mark E. Peterson said...

The initial dose of te methimazole is based on the size of the thyroid tumor and severity of the dog's hyperthyroidism (ie, dogs with larger tumors and higher T4 levels generally require higher doses of methimazole).

If long-term control of the hyperthyroidism is planned, dose adjustments are made on the basis of serum T4 values. We generally want a T4 value in the low- to mid-normal reference range (around 1.5-2.5 μg/dl is ideal).

One must always remember, however, that most hyperthyroid dogs have a malignant thyroid tumor. Controlling the hyperthyroidism with methimazole or another antithyroid drug will not slow the tumor growth or metastasis. So definitive therapy (with surgery, radiation treatment, and/or chemotherapy) will offer a better long-term prognosis than use of methimazole alone.

Eva German said...

Dr. Peterson,
my 5 years old dog's results are following: T3-1,7 (0,7-2,3 nmol/L); T4-61,6 (12-50 nmol/L); FT4- 24,1 (7,7-14,6 pmol/L); TSH- less than 0,005. Our vet thinks that she doesn't have carcinoma though the ultrasound showed the change on thyroid gland, that she described as structural change due to high hormones. What do you suggest that we do? Thank you! Eva

Dr. Mark E. Peterson said...

We can't diagnose thyroid carcinoma from blood tests (or ultrasound) but the high levels of T4 and free T4 and low levels of TSH are consistent with hyperthyroidism.

Most dogs (>90%) with hyperthyroidism have thyroid carcinoma so I'm not sure what you veterinarian meant in the explanation to you. If this was my case, I'd certainly take a chest xray looking for metastasis and consider a thyroid biopsy to definitively diagnose or exclude thyroid cancer.

Leslie Bracken said...

Dear Dr. Peterson
My 11 1/2 y.o. mixed breed dog has been diagnosed with hyperthyroidism. He had a lump slightly smaller then a ping pong ball. His test results are:
T4-8.4 ug/dL
TSH-<.03 ng/mL
Free T4 >100 pmol/L

He is currently taking Methimazole 10 mg twice per day. I cannot afford surgery to remove the tumor and I understand that 95% of these tumors are malignant. He is losing weight, drinking and urinating frequently(specific gravity of urine is 1.005), panting, vomiting bile approx twice/week.

Will the Methimazole keep him more comfortable as the cancer spreads? What is the approximate amount of time that I can expect him to live? He's very much himself at the moment and seems to be in no pain, although the symptoms above are present. I understand that an opinion like this is tough seeing as how you have not seen the dog. However, my vet is trying to push me into bordatella vaccine and more blood tests. I'm trying to decide if those things are worth doing, however, I don't know if my boy has weeks to live or potentially months. Any information you can pass on would be greatly appreciated and taken as your best educated guess and not a guarantee.

Dr. Mark E. Peterson said...

Most of these dogs will have cancer, but occasionally we will see a dog with a benign thyroid tumor or a carcinoma that has not metastasized. The prognosis depends largely on if your dog has metastasis to the lungs or other sites.

Has a chest xray been done? Are both thyroid lobes involved or just the one? Is the thyroid mass "fixed" or freely movable. If there is no spread, only the 1 lobe is involved, and it's not locally invasive, a unilateral thyroidectomy might cure him and that shouldn't really cost that much. If it's invasive, etc, then the prognosis is poorer and he may only have a few weeks to months.

Crasmussen said...

My almost two year old Maltese showed hyperthyroidism. My vet is currently checking with vets that specialize in this. She has none of the symptoms of hypothyroidism. Is there anything else thar could cause elevated levels. I took her to our vet because she would shake like she was cold. Your thoughts. CR

Dr. Mark E. Peterson said...

A thyroid tumor causing hyperthyroidism in a 2-yr old dog would be very unusual. I'd certainly verify the T4 result; and I'd recommend that your vet run an entire thyroid panel to get a better idea about what's going on (to do that, request a total T4, free T4, T3, TSH, and thyroglobulin autoantibodies).

A high T4 could be hyperthyroidism, hypothyroidism (from T4 autoantibodies), or lab error. Running the whole panel should sort that out.

Sultana Alam said...

Dogs, similar to humans, will suffer from completely different health conditions. whereas some area unit delicate and might be treated reception, others may be grave. this is often why pet house owners got to bear in mind of the signs which will indicate that one thing isn't right with their pet. See more

Dr. Mark E. Peterson said...

Call your vet--- immediately!

Cassie said...

Dr. Patterson,
I am a veterinary technician, and my cat has been very sick over the past week. She started with general malaise, progressed to lethargy, anorexia, mild PU/PD, and ataxia. CBC/chem was grossly normal, with the only abnormality was she was hyponatremic (sodium was 133 mmol/L). Her condition continued to worsen as we tested further, resting cortisol is normal, T4 is low normal (0.8), sodium dropped to 124 mmol/L; she responded to IV fluids and some mannitol and furosemide, sodium increased to 134 mmol/L, seems more bright than before, though still lethargic. Today, I palpated what seems like a mass on her thyroid. So my question is this, do you think it is possible that she could have a non-functional thyroid mass that is causing destruction of the thyroid gland and creating a hypothyroid myxedema coma? I would appreciate your feelings on this; all the doctors and staff are currently at a loss for her condition. Thank you for your time!

Dr. Mark E. Peterson said...

Highly unlikely that a tumor would cause hypothyroidism. But you could run another T4 with a TSH level. If T4 is low to low normal and cTSH is very high, that is diagnostic for hypothyroidism. Most hypothyroid cats I've seen are not been very ill or anorexic.

Cassie said...

Thank you for your reply! I'm glad to report that she is doing better, but will continue to monitor her thyroid. I appreciate your quick response!

Amber said...

Took her to the vet and she has lymphoma.. :( so prednisone and antibiotics cause ahe had a fever and her appetite has already increased and she is more like her old self but the vet said that at some poiny we will have to think about her quality of life

Nicole Toth said...

Dr. Peterson,
I'm a vet tech with an 8 year old MN dachshund. He has had a neck mass for 2 years now.
First, weight loss, pu/pd, and scruffy coat appeared. I had left him at a dog sitters for 3 weeks while i was on vacation, i increased his food, dewormed him, etc. A month later I noticed a round, firm, immovable mass on the left side of his neck. Chest x-rays were normal, blood levels all normal. He has maintained a healthy appearance since. Periodic pu/pd and mild behavior changes (in a doxie hard to distinguish!) occurred the first year.
Ultrasound imaging upon discovery was consistent with lymph node, cytology showed skin cells & WBCs. Thus, my doctors diagnosed Lymphoma. As he is very difficult to medicate and hates being at the clinic, I opted for no medical treatment. Instead I chose nutritional supplements and comfort care. Two years later the mass has grown slowly, his only symptom is that he snores and sleeps a little more.
I have him imaged again, the mass appears operable and over his left thyroid gland. His chem panel only showed a 6 point increase in Glu, Ca+ in the normal range. We did a thyroid panel- all wnl.
I have one doc telling me to take it out and he'll have 2 years. He just felt it and said it's a thyroid tumor. The other doc imaged it and wants a new cytology. And as opinions seem to be all over the board, I don't want to cut my dog unless I absolutely need to (he does not handle medical treatment well, "non-compliance" is his motto!). My questions- What is the ideal next step? On cytology, what would you expect to see? What blood work and tests would give an optimal picture of what's going on? With the tests already run, what other ones would narrow down a diagnosis?
I greatly appreciate your expertise on this matter, especially since it's part of your free time. Thank you :)

Dr. Mark E. Peterson said...

If a chest xray is clean, I'd definitely remove the mass and have it biospied.

Katherine said...

I bred and co-own a female welsh springer spaniel, now 16 months old, with bloodwork report indicating hyperthyroidism. Physical exam for tumors tomorrow. She has been underweight for months and I have encouraged increases in food and changes in diet (dry kibble fed) to bring her up to normal weight. Highly active pup since birth and a bit undersized, also since birth. Coat is now in lessor condition, and tiny rise above normal in the IDEXX SDMA values is present. Protein and cholesterol are low. Other blood numbers & WBC normal.

My own Vet has never seen a case in dogs in her 30 years of practice. I don't know about the puppy's custodial owners' DVM who we will see tomorrow, but surmise from posts on your site and others that this is very rare, and especially unusual in dog this young.

Her Free T4 is quite elevated. IDEXX results pasted below There is a great deal of hypothyroid in Welsh Springers, but I cannot find anyone who has heard of a case of hyper. Is there anything else we should test for?
Thank you,

3/22/17 (Order Received)
3/22/17 2:54 PM (Last Updated)
Total T4 2.4 1 - 4 µg/dL

Free T4 (ng/dL) >6.0 0.6 - 3.7 ng/dL

Free T4 (pmol/L) >77.2 7.7 - 47.6 pmol/L

cTSH 0.12 0.05 - 0.42 ng/mL

Dr. Mark E. Peterson said...

This is unlikely to be really hyperthyroidism -- too young of age, total T4 normal, and TSH normal (TSH should be undetectable). I'd repeat the free T4 by another method or lab.

Katherine said...

Thank you very much for the response. It was a great relief to receive! This backs up the thinking of a second Veterinarian and we are ordering a new test in a different lab, plus pursuing other possible problems.