Hyperthyroidism is the most common endocrine disease of middle to older aged cats. The thyroid glands are paired and lie adjacent to each side of the trachea (wind pipe) in the neck. Thyroid glands are usually not palpable (capable of being felt) in a normal cat. Cats with hyperthyroidism generally have a palpable enlargement of one or both thyroid glands. Most hyperthyroid cats have involvement of both thyroids (bilateral). The enlargement is usually benign resulting in excessive production and release of thyroid hormone in the blood. The most common clinical signs associated with the disease include weight loss despite a normal or increased appetite, hyperactivity, increased body temperature, increased defecation, and fast heart rate (tachycardia). The heart may have a classic gallop rhythm (horse hooves) on physical exam. Thyroid hormones also have direct effects on the heart that may require further diagnostics such as an ECG (electrocardiogram).
Diagnosis is by a blood test measuring serum levels of thyroid hormones (T4 and Free T4). Since the disease occurs most commonly in older cats, a blood chemistry is recommended to rule out the presence of other diseases. It is well known that the increased metabolic rate and blood flow through the kidneys may help, but mask coexisting kidney (renal) disease. Therefore, management and control of hyperthyroidism should also include monitoring kidney levels.
There are three treatment options for hyperthyroidism:
Thyroid medication such as methimazole (tapazole) work by blocking the production of thyroid hormones, but does not remove excess thyroid tissue. Medication must be continued daily as a lapse in therapy results in a recurrent hyperthyroid state in just a few days. Surgery removes excess thyroid tissue while radiation therapy destroys the excess thyroid tissue. Radioiodine requires referral to a facility that specializes in radiation. In addition, radiation therapy requires a prolonged hospitalization and confinement for radioactivity.
Consult your veterinarian regarding individual treatment recommendations.