Objectives

• Review classic signs of hyperthyroidism and contrast them with the typical presentation currently

• Discuss theories and evidence for these theories of the increasing incidence of hyperthyroidism

• Outline advantages and disadvantages of surgical and non-surgical therapies

Key Points

• Veterinarians are in a position to diagnose early hyperthyroidism before the damage has been done

• Total T4 (TT4) is the best screening test, but a more complete feline panel or nuclear scintigraphy may be necessary if TT4 is non-diagnostic

• Euthyroid sick syndrome is possible in cats and must be accounted for in your assessment

• I-131 is becoming more available and is the ideal permanent therapy for cats

• Methimazole (tapezole) can be used topically in difficult-to-pill patients

Clinical Evaluation

When hyperthyroidism was first reported in cats as a disease entity approximately 25 years ago, the majority of cases were advanced. The cats were thin, aggressive, polyuric, polydipsic, polyphagic and had large palpable goiters. As cats have moved out of barns where they served as 'mousers' and into peoples' homes as beloved family members, owners are expecting veterinarians to provide excellent routine health care for their cats. Routine wellness exams give veterinarians an opportunity to diagnose hyperthyroidism before many of the classic clinical signs occur. Diagnosis can be confounded by the multitude of concurrent illnesses that can affect the aging cat. You may now be confronted with the fat, happy cat that has only recently begun urinating more frequently in the litter box and has a trivial thyroid slip on physical examination. Yet this patient may be the ideal candidate for anti-hyperthyroid therapy such as I-131.

Diagnostic Workup

In addition to a thorough physical exam, all geriatric cats (8 year or older) should have annual blood work performed. A minimum database in the older cat includes complete blood count, biochemistry panel, urinalysis and a total T4. If the cat is an indoor/outdoor cat or lives in a household where new cats are being introduced, FeLV/FIV should be tested as well. Illnesses such as diabetes mellitus, renal disease, anemia, hypercalcemia and gastrointestinal disease are a few of the diseases/abnormalities which can appear later in life and which may falsely lower the total T4 in a cat that actually has hyperthyroidism. Liver enzymes may indicate concurrent hepatic disease or simply reflect the cat's hyperthyroid state, especially if only ALT is mildly elevated.

Ruling In (or Out) Hyperthyroidism

Total T4 remains the best, most inexpensive screening test to detect hyperthyroidism in cats. The time to consider additional thyroid measurements, such as free T4 by equilibrium dialysis, are in those cases in which hyperthyroidism is suspected based on clinical signs, but where total T4 is within normal limits. The gold standard for the diagnosis of hyperthyroidism is nuclear scintigraphy. The number of facilities certified to provide nuclear scintigraphy and other radioactive testing/therapy are increasing.

Therapeutic Options

In all cases of hyperthyroidism, treatment is directed at achieving a euthyroid status (T4 < 2.0) while maintaining reasonable renal function. In cases in which renal insufficiency is severe once the cat becomes euthyroid, a permanent surgical or chemotherapeutic solution is not recommended. Therefore, a "tapezole challenge" is essential for any cat whose owner is interested in pursuing surgery or I-131 therapy. For owners who prefer medical therapy, or whose pets are ineligible for permanent therapy, options include methimazole (oral), methimazole (transdermal formulations) and carbimazole. I-131 therapy is considered the ideal therapy for those cats that maintain kidney function at normal thyroid levels. Advantages of I-131 therapy include permanent therapy, minimally invasive technique, targets primarily the abnormal thyroid tissue, and almost no post-therapy complications. I-131 therapy is expensive (although the expense is comparable to surgery with intense monitoring for post-surgical hypocalcemia), requires isolation of the cat for up to 2 weeks after injection and may not be available to all general practitioners.

Topical therapy with methimazole can be an option for owners who are unable to elect a permanent therapy for their cat and who have difficulty medicating their pet. Studies have shown that it is possible to achieve therapeutic levels of methimazole in the bloodstream with transdermal formulations. Disadvantages include the greasy nature of the formulations, inconsistent application and the usual problems of compliance with life-long therapy.

Summary

Hyperthyroidism is increasingly seen in the feline population, due in part to the increasing life-span and excellent veterinary care these cats receive. Identifying and treating hyperthyroidism early requires testing for and recognizing more subtle signs of the disease. Total T4 is the preferred screening test, but early disease may be identified with further assessment of free T4 and/or nuclear scintigraphy. Non-invasive, permanent therapy, such as I-131 is becoming more available and is considered the treatment of choice. Medical therapy is still a viable option, but can now be given transdermally in those patients that do not tolerate twice-daily oral administration.

Selected References

Edinboro CH, et al. Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. JAVMA 2004;224:879-886.

Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentations and response to carbimazole therapy. Aust Vet J 2000;78:462-465.

DiBartola SP, et al. Effect of treatment of hyperthyroidism on renal function in cats. JAVMA 1996;208:875-878.

Behrend EN. Medical therapy of feline hyperthyroidism. Compend Cont Educ Pract Vet 1999;21:235-244.

Hoffmann G, et al. Transdermal methimazole treatment of cats with hyperthyroidism. J Fel Med Surg 2003;5:77-82.