by Jennifer Chaitman, VMD, ACVIM (Internal Medicine) and Eveline Han, ACVIM (Internal Medicine)
The presence of excess thyroid hormone is called hyperthyroidism. This condition is a common hormonal disease of cats who are ten years or older although rarely it can be seen in cats as young as six years old. The cause of hyperthyroidism is not known. It occurs when the thyroid glands become overactive and enlarged (hyperplastic). About 1% of cases are due to a malignant cancer of the thyroid glands.
Thyroid hormone helps regulate the animal's basic metabolic rate and has strong influences on almost every organ in the body with regards to normal balance of metabolism/function (homeostasis). Cats that develop hyperthyroidism may have clinical signs of gradual weight loss, good or increasing appetite, poor haircoat, diarrhea, increased thirst, panting, fast heart rates, and aggressive play behavior. Hyperthyroid cats can also present as tired, depressed, and with loss of appetite in about 10% of cases (apathetic hyperthyroidism). In addition problems with the heart (hypermetabolic or hyperdynamic function) can involve thickening of the heart muscle and poor function (hypertrophic cardiomyopathy). Other metabolic diseases such as diabetes can be harder to control if the thyroid glands are overactive. High blood pressure can be present (hypertension) which can be damaging to the kidneys, brain, eyes, and heart. Hyperthyroidism if untreated can lead to heart failure, fluid accumulation in the chest or abdomen, kidney disease, stroke-like episodes, and severe loss of body condition.
Laboratory clues can include elevations in liver enzyme values in the serum. The tests for hyperthyroidism can be simple or complex. The first test is often a serum "total" T4. If this is confirmatory then no further confirmation may be needed. If it does not confirm the problem, but based upon your veterinarian's assessment hyperthyroidism is still a consideration, then other tests such as a "free" T4 (by equilibrium dialysis) or a T3 suppression test may be advised. Most but not all hyperplastic thyroid glands are palpable by your veterinarian. But some palpable nodules in the thyroid area are due to scar tissue, lymph nodes, and non-functional thyroid nodules.
The options for treatment are evaluated based upon your pet's health status, risks, costs, and age. A drug called methimazole (Tapazole) is often used initially to suppress production of thyroid hormone. Although frequently effective, and less expensive, it can cause adverse drug reactions that are mild (loss of appetite) to moderate (severe facial itchiness) to life threatening (bone marrow disease). This can be used for a few weeks to stabilize your pet until another option is chosen/appropriate, or can be continued for years if there is frequent monitoring for adverse effects. Surgical removal of affected thyroid nodules is also an efficient method of treatment but may need to be repeated (recurrence in another area of the thyroid gland) or fail to find the gland if it is in the chest (5% of cases). The most recommended choice is the use of a radioactive iodine compound (I-131) which is injected under the skin. It is safe and effective, resulting in a cure in 95% of cats that are treated. However, it does require hospitalization for several days. Cost for this is higher initially, but no different over months to years of use of methimazole medication. Jennifer Chaitman, VMD, Dipl. ACVIM will offer guidance in the appropriate selection of treatment for hyperthyroidism. Monitoring of your pet's general health and response to medications is essential. Blood tests, cardiac evaluations, blood pressure checks, and urine samples will be advised as needed. The prognosis for controlled hyperthyroidism is good.