Hyperthyroidism in Cats: Past and Upcoming Topics

As you know, if you have been following this blog, I've spent much of the last few months writing about issue of hyperthyroidism, the most common endocrine disorder of the cat.

Unfortunately, I got sidetracked and have not yet finished discussing all of the treatment options that we have for management of this common problem. However, I do plan to finish up this series of topics over the upcoming weeks.

But before I move on to the topics of treatment of this common feline condition, I thought I'd post links to the topics I've covered thus far:

1. Do All Hyperthyroid Cats Have a Thyroid Tumor? Is It Thyroid Cancer?
2. Why Has Hyperthyroidism in Cats Reached Epidemic Levels?
3. Top 10 Signs of Hyperthyroidism in Cats
4. Top 12 Physical Exam Findings in Cats with Hyperthyroidism
5. Hypertension (High Blood Pressure): A Common Problem in Cats
6. Diagnosing Hyperthyroidism in Cats: Routine Testing Procedures
7. Diagnosis of Hyperthyroidism in Cats: Serum T4 Concentrations
8. Diagnosis of Hyperthyroidism in Cats: Serum T3 Concentrations
9. Diagnosis of Hyperthyroidism in Cats: Serum Free T4 Concentrations
10. Diagnosis of Hyperthyroidism in Cats: Serum Free T4 (Part 2)
11. Diagnosis of Hyperthyroidism in Cats: Serum Free T3 Concentrations
12. Diagnosis of Hyperthyroidism in Cats: Serum TSH Concentrations
13. Diagnosis of Hyperthyroidism: T3 Suppression Test
14. Diagnosis of Hyperthyroidism: TRH Stimulation Test
15. Diagnosis of Hyperthyroidism: Thyroid Scintigraphy
16. Treatment Options and Considerations for Hyperthyroid Cats
17. Do Hyperthyroid Cats Ever Go Into Spontaneous Remission? 
18. Daily Water and Fluid Requirements and Needs for Hyperthyroid Cats
19. Diet and Nutritional Management of Hyperthyroid Cats
20. Treating Cats with Hyperthyroidism: Antithyroid Drugs

I'll be posting my next post on feline hyperthyroidism (entited, Antithyroid Drug Treatment for Hyperthyroidism: Brand Name, Generic, or Compounded Drug?) within the next couple of days; then I'll continue with treatment issues, with about a post once a week.

Can a New Thyroid Health Food (Hill's y/d) Live Up to the Hype?

Can a New Thyroid Health Food Live Up to the Hype?

A new prescription cat food promises to treat hyperthyroidism in cats. 
Maybe so, if the studies pan out. Where are those studies, anyway?


Dr. Eric Barchas
www.catster.com
May 3, 2012

In the blog post entitled, Can a New Thyroid Health Food (Hill's y/d) Live Up to the Hype?, Dr. Eric Barchas provides us with a good overview of what's wrong with all the hype associated with Hill's y/d diet.

As Dr. Barchas states in the beginning of his post (1): "For months, every veterinary publication I receive has been plastered with full-page advertisements for a new prescription cat food...... The new cat food is called Hill's y/d —or, more formally, Hill's® Prescription Diet® y/d™ Feline Thyroid Health on the company's website. Call me old-fashioned, but I am naturally suspicious of any food that has two ®s and a ™. Might the diet be more marketing ploy than medical breakthrough?"

For those of you who have followed my rants about what's wrong with Hill's y/d diet (2,3), you would be able to predict that this post made me smile.

So what's this diet all about and what's the problem with it? 

First of all, Hill's y/d diet certainly does offers another treatment option for hyperthyroid cats. The basis for how the diet works to control the hyperthyroid state is by severely limiting the amount of iodine in the y/d (the diet is actually iodine deficient).

Because thyroid hormone contains large quantities of iodine, the diet should reduce secretion of the hormone. Indeed, the clinical experience of practicing veterinarians to date indicate the this diet is effective in lowering serum T4 values in many hyperthyroid cats, although not always to levels that we would consider "low enough."

The biggest concerns are in regard to the long-term side effects of feeding an low iodine or iodine deficient diet to cats. For y/d to work, no other food can be fed for the rest of the cat's life — so the y/d diet, if successful, would potentially be fed for years to a cat suffering from hyperthyroidism.

As Dr. Barchas points out in his post, large, long-term safety studies are needed to determine what happens when cats with or without thyroid disease eat y/d months to years. Now I know it seems difficult to believe, but adequate long-term safety studies of cats fed this diet have yet to be done.

Because Hill's y/d is not a drug (although it's certainly being marketed as a replacement for methimazole), the company is not required to do long-term safety studies. And it's fairly clear that they have no intention of paying to have them done.

The bottom line: 

Again, Dr. Barchas says it better than I could do so let me quote the summary for his post (1): "In short, I believe that Hill's has unleashed y/d with too little research and too much hype. The food may turn out to be a veritable godsend, or it could be a disaster. Only time, and more studies, will tell.... 


For now, no matter how many fancy ads I see, I am hesitant to recommend the food unless a cat absolutely is not a candidate for any of the other three hyperthyroidism treatments."

Related links: 

1. To read the complete post by Dr. Barchas, click this link. 
2. To read my posts on y/d on the Animal Endocrine Clinic blog, click this link.
3. To read my posts on y/d on the Insights into Veterinary Endocrinology blog, click this link.

Treating Cats with Hyperthyroidism: Antithyroid Drugs

In cats, hyper­thyroidism can be treated in four ways — chronic administration of an antithy­roid drug, surgical thyroidectomy, radioactive io­dine (131-I), or lifelong feeding of an ultra-low iodine diet.

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated heart or kidney dis­eases or other major medical problems, availability of a skilled surgeon or radioiodine treatment facility, and owner's preference (1-4).

In this post, I’m going to discuss the use of antithyroid drugs for treating cats with hyperthyroidism. This is the most common means that veterinarians use to treat this common condition, so let’s start by discussing the pros and cons of this form of treatment.

The Advantages and Disadvantages of Antithyroid Drugs

Advantages
Chronic management with antithyroid drugs is a practical treatment option for many cats with hyperthyroidism, and offers many advantages. Medical management requires no special facilities and can be prescribed by all veterinarians (1-4). These drugs cause a rapid fall in serum thyroid hormone levels (i.e., the high serum T4 normalizes within 1-3 weeks), which may be desirable in severely affected hyperthyroid cats (5,6).

Anesthesia is avoided, as are the surgical complications associated with thyroidectomy (I’ll be discussing surgical treatment in my next post). In contrast to surgery or radioiodine treatment, hospitalization is not required with medical treatment. Finally, the initial, upfront costs of antithyroid drugs is much less than with either surgical or radioactive iodine treatment.

Disadvantages
Long-term medial management also has many disadvantages. This form of treatment is not curative, is highly dependent on owner and cat compliance, and requires regular biochemical monitoring to ensure the efficacy of treatment (1-4). Side effects are common, occurring in up to 20% of cats (1-6). Even though the initial cost of medical treatment may be far less initially, the cost of ongoing monitoring over a period of months to years can exceed that of thyroidectomy or radioiodine therapy.

These antithyroid drugs also come with other drawbacks. Since they block thyroid hormone synthesis but do not destroy the cat’s thyroid tumor, these drugs never cure the hyperthyroidism and relapse will always occur if daily medication is discontinued (1-6). Most importantly, the benign thyroid tumor — which is present in all cats with hyperthyroidism (7,8) — continues to grow and, after many months, may transform from adenoma to thyroid carcinoma in some cats (9).

Long-term medical management is best reserved for cats of advanced age or for those with concurrent diseases, and for when owners refuse either surgery or radioactive iodine. In addition to long-term treatment, medical management is also advised prior to surgical thyroidectomy to decrease the metabolic and cardiac complications associated with hyperthyroidism. Short-term medical management is often recommended as trial therapy to determine the effect of restoring euthyroidism on kidney function, especially in cats with suspected chronic kidney disease (1-4,10).

Methimazole and Carbimazole: The 2 Antithyroid Drugs

The two drugs methimazole and carbimazole are commonly recommended for managing cats with hyperthyroidism (1-6). A related drug, propylthiouracil, often used in human medicine, is not recommended for cats because of a high incidence of serious adverse reactions —especially anemia and bleeding problems (11).

Methimazole blocks thyroid hormone synthesis by inhibiting thyroid peroxidase, an enzyme involved in the oxidation of iodide to iodine, incorporation of iodine into thyroglobulin, and coupling of tyrosine residues to form T4 and T3 (12). Methimazole does not block the release of preformed thyroid hormone, so there is a delay of 1 to 3 weeks before serum T4 concentrations return to normal after initializing therapy (5, 6).

Carbimazole is a pro-drug of methimazole. That means that after oral administration of carbimazole, it is almost immediately converted to methimazole by the cat (13). So basically, it turns out that carbimazole and methimazole end up being the same drug (1-4).

Different Drug Formulations of Methimazole & Carbimazole

Methimazole Tablets
Methimazole is specifically licensed for treatment of feline hyperthyroidism both in the USA and Europe as 2.5- and 5-mg tablets (Felimazole, Dechra Veterinary Products). It is also available as a generic and brand name drug for human use (Tapazole). For most hyperthyroid cats, a starting dose of 1.25 mg to 2.5 mg methimazole is administered twice daily is recommended (1-4,14).

Carbimazole Tablets
Carbimazole is available for human use in many European countries (brand name, NeoMercazole), Australia and Japan (2-4). This drug is not available as a licensed drug in the USA, but it is available though compounding pharmacies.

As noted above, carbimazole exerts its antithyroid effect through immediate conversion to methimazole when administered orally (13). Serum concentrations of methimazole achieved after carbimazole administration are less than after a similar weight of methimazole such that a 5-mg dose of carbimazole is approximately equal to 3 mg of methimazole (3,6). Because of that, a starting dose of regular carbimazole of 2.5 mg to 5 mg twice daily is commonly recommended for restoring euthyroidism (2-4,6).

Carbimazole is often touted as having a lower incidence of adverse reactions such as vomiting and anorexia (6,15). This may be because it is tasteless whereas methimazole has a bitter taste (3,6). However, Felimazole, as licensed for veterinary use, is sugar-coated; provided the tablet is not crushed, the bitter taste is presumably avoided.

Carbimazole Tablets (Controlled-Release)
A controlled-release formulation of carbimazole (Vidalta, Intervet Schering Plough) is licensed for cats in Europe for once daily administration (16,17). This formulation is not available in the USA.

Administration of this drug with food significantly enhances its absorption (16). The starting dose for controlled release carbimazole is 15 mg administered once daily. In cats with mild hyperthyroidism (total T4 concentration <100 nmol/L or < 8 μg/dl), a 10 mg once daily is recommended (2-4,16).

Transdermal Antithyroid Drugs
Carbimazole and methimazole can be reformulated by a veterinary compounding pharmacy and applied to the non-haired inner portion of a cat’s pinnae (ear lobe) for transdermal administration (18-20). Such custom formulation increases expense of therapy and the stability of the product can never be guaranteed.  To prevent absorption of the drug through one's own skin, it is best to wear gloves or a finger cot for application, and wash your hands afterwards.

Both antithyroid drugs are generally effective in cats when administered at a dose of 1.25 mg to 2.5 mg twice daily transdermally (2-4).  One advantage of using a compounded formulation of methimazole (or carbimazole) over the 2.5- or 5-mg tablets is that it is easier to make smaller or finer dose adjustments.

Transdermal administration is associated with fewer gastrointestinal side effects than the oral route (19,21,22), but some cats resent manipulation of their ears and crusting can occur between doses leading to erythema. These problems can usually be prevented by removing any crusted material and cleaning the ear flap prior to administration.

Monitoring of Hyperthyroid Cats on Antithyroid Drug Treatment

Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily (1-4,21-23).

Initial Monitoring
Initially, cats should be reassessed after 2 to 3 weeks and a serum total T4 concentration measured. When monitoring, time of serum T4 sampling in relation to the administration of the antithyroid drug is not overly important (1-4,24). The goal of medical therapy is to maintain total T4 concentrations within the lower half of the reference range (1-4). Low serum T4 values should be avoided, however, because it has been shown that chronic hypothyroidism is deleterious to kidney function and may worsen already present chronic kidney disease (25,26).

If hyperthyroidism persists during antithyroid drug treatment, lack of owner or cat compliance should always first be eliminated as a reason for the failure of therapy. If the serum T4 concentrations remain high with proper treatment, however, the daily dose of methimazole or carbimazole can be increased in 2.5-mg increments, reassessing the cat again in 3 to 4 weeks (1-4).

Long-Term Monitoring and Treatment
For chronic management (once euthyroidism has been achieved), the daily antithyroid drug dosage is adjusted to the lowest possible dose that effectively maintains euthyroidism. Once the dosage has stabilized, the cat should be monitored every 3 to 6 months and as needed clinically. At time of each of these rechecks, a complete physical examination should be perform together with determination of a complete blood count, serum chemistry profile, and serum T4 concentration.

Relapses are common in cats treated with an antithyroid drug. Some cats will become more difficult to medicate over time, whereas others will need higher daily drug dosages to inhibit thyroid hormone secretion as their thyroid tumors continue to grow larger and larger (27).

It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L). Recent research indicates that hyperthyroidism may contribute to the development or progression of chronic renal disease in cats (28-30).  Leaving a hyperthyroid cat untreated (or poorly regulated with methimazole or carbimazole) may therefore be detrimental to long-term kidney function and is never recommended.

During long-term treatment, it is again important to avoid inducing hypothyroidism, which may be deleterious to the cat’s kidney function (25,26). If hypothyroidism is suspected (which can develop even if the T4 is low-normal), a complete thyroid panel is recommended, including determination of the serum concentrations of total T4, free T4, T3, and TSH (see my previous blog posts on diagnostic testing for more information about these tests). The findings of low serum free T4 with high TSH concentrations is diagnostic for iatrogenic hypothyroidism; in those cats, the daily dose of methimazole  should be decreased.

Because antithyroid medications have no effect on the underlying lesion, the thyroid nodules continue to grow larger and larger over time. This may necessitate an increased daily dose with time. In some cats, large enough dosages can no longer be administered to control the hyperthyroidism and surgery or radioiodine is needed to control the hyperthyroidism (27).

Side Effects & Adverse Reactions

Most clinical adverse reactions occur within the first 3 months of therapy (1-4). Mild clinical side effects of vomiting, anorexia, or depression occur in approximately 10-15% of cats, usually within the first 3 weeks of therapy (4,5). In most cats, these reactions are transient and do not require permanent drug withdrawal.

Mild Side Effects

Figure 1: Facial excoriations
due to methimazole

Early in the course of drug therapy, mild and transient hematological abnormalities, including leucopenia (low total white blood cell count), lymphocytosis (high lymphocyte count), or eosinophilia (high eosinophil count) develop in up to 15% of cats without any apparent clinical effect (1-6).

Self-induced excoriations of the head and neck (from scratching) occasionally develop, usually within the first 6 weeks of therapy (see Figure 1).

Less commonly, generalized enlargement of lymph nodes may develop during drug treatment (31). If either of these adverse effects occur, the drug must be stopped and another form of treatment given.

Life Threatening Side Effects

Figure 2: Bleeding from gums due
to methimazole

More serious hematological complications occur in less than 5% of cats and include a severe lowering of the white blood cell count (agranulocytosis) or platelet count (thrombocytopenia).  Liver dysfunction, characterized by marked increases in all hepatic enzymes, develops in less than 2% of cats (1-5).

Rarely, cats may also develop a severe bleeding tendency during drug treatment (see Figure 2) (5,32). All of these adverse effects are reversible upon discontinuation of the medication.

The Bottom Line

Short-term or chronic management with antithyroid drugs provides a useful treatment option for many cats with hyperthyroidism. However, this treatment does not cure the disease and requires daily medication for the rest of the cat's life.  Like any prescription drug, methimazole or carbimazole can produce adverse side effects, which may be life-threatening is some cats. For all of these reasons, antithyroid drugs will never be the "treatment of choice" for all cats with hyperthyroidism.

References

1. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Barcelona, Multimedica, 2008; 127-168.
2. Baral R, Peterson ME. Thyroid diseases. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; in press.
3. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
4. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
5. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157. 
6. Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. Journal of Small Animal Practice 1992;33:228–235. 
7. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
8. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
9. 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.
10. Becker TJ, Graves TK, Kruger JM, et al. Effects of methimazole on renal function in cats with hyperthyroidism. Journal of the American Animal Hospital Association 2000;36:215–223. 
11. Peterson ME, Hurvitz AI, Leib MS, Cavanagh PG, Dutton RE. Propylthiouracil-associated hemolytic anemia, thrombocytopenia, and antinuclear antibodies in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1984;184:806-808. 
12. Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
13. Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355. 
14. Plumb DC. Plumb's Veterinary Drug Handbook (7th Ed). PharmaVet Inc, Stockholm, Wisconsin 2011.
15. Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy. Australian Veterinary Journal 2000;78:462-465. 
16. Frénais R, Burgaud S, Horspool LJ. Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats. Journal of Veterinary Pharmacology and Therapeutics 2008;31:213-219.
17. Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. Journal of Small Animal Practice 2009;50:510-515. 
18. Hoffman S, Yoder A, Trepanier L. Bioavailability of transdermal methimazole in a pluronic lecithin organogel (PLO) in healthy cats. Journal of Veterinary Pharmacology and Therapeutics  2002;25:189-193. 
19. Hoffman SB, Marks SL, Taboada J et al. Transdermal methimazole treatment in cats with hyperthyroidism. Journal of Feline Medicine and Surgery 2003;5:77–82. 
20. Buijtels JJ, Kurvers IA, Galac S et al. Transdermal carbimazole for the treatment of feline hyperthyroidism, Tijdschrift voor Diergeneeskunde 2006;131:478-482. 
21. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
22. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
23. Trepanier LA, Hoffman SB, Knoll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 2003;222:954–958. 
24. Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. Journal of Veterinary Internal Medicine 2009;23:1025-1030. 
25. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 2010;24:863-869. 
26. Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092. 
27. 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.
28. Lapointe C, Bélanger MC, Dunn M, et al. N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism.  Journal of Veterinary Internal Medicine 2008;22:1103-1110. 
29. van Hoek I, Lefebvre HP, Peremans K, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domestic Animal Endocrinology 2009;36:45-56.  
30. van Hoek I, Meyer E, Duchateau L, et al. Retinol-binding protein in serum and urine of hyperthyroid cats before and after treatment with radioiodine. Journal of Veterinary Internal Medicine 2009;23:1031-1037. 
31. Niessen SJ, Voyce MJ, de Villiers L, et al. Generalised lymphadenomegaly associated with methimazole treatment in a hyperthyroid cat. Journal of Small Animal Practice 2007;48:165-168. 
32. Randolph JF, DeMarco J, Center SA, et al. Prothrombin, activated partial thromboplastin, and proteins induced by vitamin K absence or antagonists clotting times in 20 hyperthyroid cats before and after methimazole treatment. Journal of Veterinary Internal Medicine 2000;14:56-59. 

Treatment Options and Considerations for Hyperthyroid Cats

The underlying cause of the thyroid tumors (adenomatous hyperpla­sia, adenoma, or carcinoma) associated with feline hyperthyroidism is not known. Because sponta­neous remission of hyperthyroidism in cats does not occur, the aim of treatment is to control the excessive secretion of thyroid hormone from the adenomatous thyroid gland.

The 4 Treatment Options 

In cats, hyper­thyroidism can be treated in four ways — surgical thyroidectomy, radioactive io­dine (131-I), chronic administration of an antithy­roid drug, or lifelong feeding of an ultra-low iodine diet. Each form of treatment has advantages and disadvantages, which must be considered when formulating the best treatment plan for the individual hyperthyroid cat.

Formulating the Best Form of Treatment for Each Cat

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated cardiovascular dis­eases or other major medical problems (e.g., chronic kidney disease), availability of a skilled surgeon or radioiodine treatment facility, and owner's preference.

The cost of therapy is a major consideration for many cat owners. Medical treatment or use of low-iodine dietary therapy costs far less initially. However, the cost of ongoing monitoring can exceed that of thyroidectomy or radioiodine therapy over a period of many months to years.

Cure versus Control of Hyperthyroidism

Of the four forms of treatment avail­able, it must be emphasized that only surgery and radioactive iodine remove and destroy the thyroid tumors, respectively, and thereby "cure" the hyper­thyroid state.

Use of an antithyroid drug (e.g., methimazole or carbimazole) will block thyroid hormone synthesis. And chronic feeding of an iodine deficient diet will reduce the production of T4 and T3 by the thyroid tumor. However, because antithyroid drugs and low-iodine diets do not de­stroy the hyperthyroid cat’s thyroid tumor(s), hyperthyroidism will recur once antithyroid drugs or dietary therapy is discontinued.

Outline of Treatment

Over the next few weeks, I’ll be discussing each of the four treatment options for hyperthyroidism in some detail. In my next post, however, l will review a frequently neglected but very important part of any of the treatment options — that is, providing the hyperthyroid cat with proper nutrition and adequate amounts of water.

Hyperthyroid cats develop many muscle wasting as well as many metabolic complications because of their disease. Therefore, they have special dietary needs and require a high-quality nutrients in their daily diet including adequate amounts of water, protein, fat, vitamins, and minerals.

References

1. Birchard SJ, Peterson ME, Jacobson A:  Surgical treatment of feline hy­perthyroidism:  Results of 85 cases.  Journal of the American Animal Hospital Association 1984;20:705-709.
2. Peterson ME, Kintzer PP, Hurvitz AI: Methimazole treatment of 262 cats with hyper­thyroidism. Journal of Veterinary Internal Medicine 1988;2:150-157.
3. Welches CD, Scavelli TD, Matthiesen DT, Peterson ME: Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Veterinary Surgery 1989;18:392-396.
4. Peterson ME, Becker DV: Radioiodine treatment of 524 cats with hyperthyroidism. Journal of the American Veterinary Medical Association  1995;207:1422-1428.
5. Peterson ME: Radioiodine treatment for hyperthyroidism. Clinical Techniques in Small Animal Practice 21:34-39, 2006
6. Peterson ME: Hyperthyroidism, In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Fifth Edition). Philadelphia, WB Saunders Co. 2000; pp 1400-1419.
7. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008, pp 127-168.
8. Peterson ME: Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC  (eds): Current Veterinary Therapy XIIII.  Philadelphia, Saunders Elsevier, 2009, pp 180-184.
9. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press
10. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012; in press.
11. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.

Do All Hyperthyroid Cats Have a Thyroid Tumor? Is It Thyroid Cancer?

Yes, all hyperthyroid cats have thyroid tumors – every last one.  However, if your cat is hyperthyroid, there is no need to panic!  Most of the thyroid tumors that cause hyperthyroidism in cats are benign (non-cancerous), so it is very unlikely that your cat has cancer, especially if the hyperthyroid condition was only recently diagnosed.

All thyroid tumors in hyperthyroid cats are made up of thyroid cells that grow and function at an uncontrolled rate. This makes your cat’s thyroid gland grow larger and larger with time.  In almost all hyperthyroid cats, the veterinarian can palpate the thyroid tumor (Figure 1).  In contrast, the veterinarian should not be able to feel a normal cat’s thyroid gland, much less see it! 

Figure 1: A hyperthyroid cat with an extremely large thyroid tumor.

In addition to making the thyroid larger, the tumor cells function at an increased rate and produce too much of the thyroid hormone thyroxine (T4).  This excess thyroid hormone in your cat's circulation is what causes the signs of hyperthyroidism, such as increased appetite, hyperactivity, and weight loss.

Thyroid tumors come in two varieties, adenoma and carcinoma.   Adenomas are by far the majority (97-98%) of tumors that cause hyperthyroidism in cats.  Adenomas are benign tumors; they do not spread (metastasize) to other parts of the body.  The normal feline thyroid gland is made up of 2 small, separate thyroid lobes, both lying adjacent to the trachea (windpipe).  In 70% of hyperthyroid cats, adenoma infiltrates both thyroid lobes (Figure 2), whereas 30% of cats have only one thyroid lobe affected.

Figure 2: Hyperthyroid cat with two thyroid adenomas at time of surgery. In this cat, the left thyroid lobe (top of image) is twice normal size, whereas the right lobe (bottom of image) is 4-6 times normal size. Both thyroid lobes were palpable on physical examination.

In 2-3% of cats, the hyperthyroid condition is caused by thyroid carcinoma, rather than thyroid adenoma.  Unlike adenomas, thyroid carcinomas are cancerous and generally grow to a massive size (Figure 3).  They are malignant tumors – carcinomas invade and metastasize to surrounding tissues, such as the chest cavity, and endanger the cat’s life.

Figure 3: Hyperthyroid cat a large thyroid carcinoma at time of surgery. In this cat, the left thyroid lobe is approximately 50 times normal size. These tumors are invasive and generally inoperable.

Although cats with thyroid carcinomas are harder to treat, radioiodine therapy is the best option.  By administering an ultra-high dose of radioiodine (e.g. up to 10 times the I-131 dose appropriate for an adenoma), it is easily possible to destroy all of the cat’s malignant thyroid tissue.  With this treatment, we can cure the cat of both their cancer and hyperthyroidism!

One question remains: what causes the 2 to 3% of hyperthyroid cats to develop carcinomas?  Again, most cats that have carcinomas have been hyperthyroid for a long time (on the scale of many months to years).  Recent evidence indicates that the tumors change over time in these cats, transitioning from normal tissue to fast growing tissue, soon turning into an adenoma.  When left untreated, or treated only with an anti-thyroid drug (see next week’s article), tumor growth continues and eventually the adenoma tissue transforms into a malignant carcinoma.

Has Your Cat Become Irritable or Aggressive? Your Cat Could be Hyperthyroid!

Whenever an adult or aging cat shows a sudden change of attitude and becomes aggressive, hyperthyroidism must be high on the list of differential diagnoses. The classic symptoms associated with feline hyperthyroidism include weight loss despite an increase in appetite. Gastrointestinal upsets (such as vomiting or diarrhea) and increased thirst are also relatively common signs. Hyperthyroid cats commonly exhibit periodic restlessness, nervousness, hyperactivity, or hyperexcitably. Less commonly, they can become irritable and clearly aggressive.



Why do hyperthyroid cats develop these unpleasant behavioral signs?

In human patients, hyperthyroidism has long been known to produce a variety of psychiatric disturbances.  Human patients with hyperthyroidism commonly exhibit anxiety, nervousness, and hyperkinesia; they commonly develop an inability to sit still, continuously tapping their fingers, feet, or shifting their position frequently. They may experience insomnia, cognitive problems, phobias, panic attacks, and psychosis.  Irritability and emotional lability are common findings in these patients. They may lose their temper easily, burst into tears, and become hysterical without any apparent reason or justification.

Clearly, many of these psychiatric disturbances are difficult to recognize or characterize in cats.  As mentioned above, hyperthyroid cats are frequently restless and appear agitated. Pacing, circling or aimless wandering may occur; this may be similar to the confusion or anxiety that develops in human hyperthyroid patients.  The cats generally sleep less deeply and are more easily aroused than normal cats.  Other behavioral signs that can develop include increased vocalization, loss of litter box training (particularly defecating outside of the litter box), and diminished affection. 

Over two-thirds of hyperthyroid cats are reported to be nervous and hyperactive, and over half of the cats appear easily prone to “stress” or panic attacks. Up to 20% of hyperthyroid cats become extremely irritable, resist being held or touched, and can be outwardly aggressive.  Some cats, pumped up on thyroid hormone, develop a low tolerance for sudden touch or sound, spurring them to attach their owners or other pets.

Surprisingly, most owners do not bring their hyperthyroid cat to the veterinarian with the primary complaint of a behavioral problem, even when aggression is present.  Most of these cats remain basically very sweet when not under stress, so the development of severe aggression in the stress-free and safe environment of the cat’s house is extremely rare.

In general, severe aggressive behavior in hyperthyroid cats is most commonly precipitated by the stress of a visit to the veterinarian’s office. These cats may become aggressive and/or hysterical with handling and physical restraint required for the physical exam or blood collection. Therefore, the veterinarian is more likely to experience this aspect of the disease than the cats’ owners would at home. 

While behavioral concerns are not the usual presenting complaint, there are exceptions, and such was the case with Mattie, a 14-year old female domestic short-hair cat.   Most of the time Mattie behaved much as she always had, but every so often she would start to tear around the apartment, completely out of control.  At such times, she was most likely to attack both the other cats in the apartment as well her owners or human guests.  These attacks were typically quite vicious, and on several occasions she had drawn blood.  The only other abnormality the owner had noticed was that she had lost weight despite the fact that her appetite had increased. There was occasional vomiting, but no diarrhea, respiratory signs, or increased thirst.

On physical examination, Mattie’s body condition score was normal, weighing in at 9.5 pounds, but there was evidence of mild to moderate muscle wasting.  Initially, she tolerated physical examination fairly well with mild restraint.  Abnormalities included a rapid heart rate, mild heart murmur, and an enlarged thyroid tumor was palpable.  Near the end of the exam, however, Mattie showed us that the handing and restraint involved was more than she could tolerate; she became hysterical and attempted to bite and scratch the veterinary assistant. Fortunately, we were still able to collect a blood sample, and the subsequent finding of a high serum T4 concentration confirmed the diagnosis of severe hyperthyroidism.

Mattie was admitted to our Hypurrcat unit and treated with radioiodine.  After 2 weeks, Mattie's T4 had normalized and she was much calmer. Over the next month, the owners reported that Mattie’s hyperexcitability, agitated mental state, and aggressive behavior all resolved.

By far the most common medical cause of sudden aggression in cats is hyperthyroidism. So if you have an older cat that is showing behavioral issues of hyperactivity, irritability, or aggression, especially if your cat is losing weight despite a good or increased appetite, hyperthyroidism should be always ruled out. Treating hyperthyroidism with radioiodine, surgery, or antithyroid drugs resolves the behavioral problems within a few weeks in most cases. This allows the cat to return to their previous, calm, peaceful zen-like existence, sometimes napping for 16 or more hours a day! 

Does My Cat Have Hyperthyroidism?

What is hyperthyroidism?

Hyperthyroidism is an extremely common disease of the middle-aged and older cat. It is a multi-system disorder caused by an increase in the amount of thyroid hormones (called T3 and T4) produced by an enlarged thyroid gland. It was first documented in cats 31 years ago but the cause of the disease has been elusive. Although the enlargement in the thyroid gland is caused by one or more tumors, these thyroid tumors are usually non-cancerous or benign tumors (adenoma or adenomatous hyperplasia) of the thyroid gland. Carcinoma (cancer) is a rare cause of hyperthyroidism in cats, and tends to develop most common in hyperthyroid cats treated for months to years with medical treatments, such as methimazole.

What are the symptoms that a hyperthyroid cat could exhibit?

Excessive thyroid hormone secretion accelerates most bodily processes and gradually causes one or more clinical signs that are apparent to the cat owner. The most consistent finding is weight loss, secondary to the increased rate of metabolism. The cat tries to compensate for this by eating more (increased appetite - see Figure 1).

Figure 1

In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! (Figure 2) 

Figure 2

Despite the increased intake of food, however, most cats lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred, or it may happen more quickly.

Other signs that the hyperthyroid cat may show include anxiety or nervousness, a rapid, pounding heart rate, muscle weakness, vomiting, diarrhea, increased thirst or urination, panting (Figure 3), and an unkempt hair coat (Figure 4).

Figure 3

Figure 4

Over time, untreated hyperthyroidism leads to deleterious effects on many of the cat's internal organs. Heart disease, muscle wasting, chronic emaciation, and/or severe metabolic dysfunction can develop, ultimately leading to death.

What should you do if you think your cat could be hyperthyroid?

If you think that your cat may have hyperthyroidism, you should make an appointment for a check-up with your veterinarian. On physical examination, your vet may feel an enlarged thyroid nodule in the neck (Figure 5), a finding that is consistent with hyperthyroidism.  

Figure 5

If hyperthyroidism is still suspected based on your cat’s symptoms and exam findings, your veterinarian will order a complete blood count and blood chemistry panel to evaluate overall body health, as well as a thyroid hormone (T4) value.  The vet may also order a chest x-ray and an ultrasound to be thorough.

Most hyperthyroid cats will have high levels of the thyroid hormone T4 in their bloodstream, confirming hyperthyroidism   In a few cats, especially those with early disease or concurrent problems, more specialized thyroid testing or a thyroid scan may be needed to confirm the diagnosis.

Once hyperthyroidism has been confirmed, there are several treatment options. They include treatment with radioactive iodine (I-131), surgical removal of the gland (thyroidectomy), and treatment with anti-thyroid medications.  Overall, after treatment, the prognosis is good to excellent depending on your cat’s overall condition and treatment success.