What is Hyperthyroidism?
Hyperthyroidism, by definition, is the excess production and secretion of thyroid hormone (T3 and T4) by the thyroid gland. It is now considered to be the most common endocrine disorder in middle to older aged cats (average age 13 years). In cats, this is usually caused by a benign thyroid tumor (adenoma or adenomatous hyperplasia) either on one or both of the thyroid glands. Rarely (1-3% of cats), hyperthyroidism is caused by a malignant thyroid tumor (carcinoma).
How do I know if my cat is affected?
The most common symptoms observed in hyperthyroid cats include weight loss despite an increased appetite, increased thirst and urination, restlessness or increased anxiety, vomiting, diarrhea, and changes in skin or hair coat. There are blood tests to determine the amount of circulating thyroid hormone in your cat.
Why is it important to treat?
Thyroid hormone is essential for fueling every cell in the body, however, too much of a good thing can be detrimental on every body system. Eventually, the body cannot keep up with the increased metabolic demands which can result in multiple organ failure when the body can no longer compensate.
What are the current treatment options?
The goal of treatment is to reduce the amount of circulating thyroid hormone in the body. There are oral medications that block the synthesis of active hormone in the body, but this is not curative and requires medicating the cat daily for life. Only 85-87% of cats respond to oral anti-thyroid medications and up to 15% can have side effects including vomiting, diarrhea, bone marrow changes, and skin lesions. Surgery can be done to remove the thyroid gland(s), but healthy tissue is also removed and the parathyroid glands are at risk for being removed due to their close proximity to the thyroid glands. Aside from being an increased anesthetic risk (older, highly stressed, and often very skinny cats with rapid heart rates and often compromised kidneys), there are many nerves in that region that have the potential for damage. Often, both glands need to be removed (70-90%) and then these animals need to have thyroid hormone supplementation. Radioactive Iodine (I131) is the safest and most effective treatment modality currently available with a success rate of 96-98%.