Hyperthyroidism is excess production of the thyroid hormone from an abnormal thyroid gland. It is the most common endocrine disease of cats. Cats have two thyroid lobes. They are on each side of the trachea [wind pipe]. Feline hyperthyroidism is most commonly from adenomatous hyperplasia [or less frequently adenoma] of one [30% of cases] or both [70% of cases] thyroid lobes. Malignant cancer [thyroid carcinoma] is rare [less than 2%].
Most are middle aged to older cats [average age of onset is 12 to 13 years old]. There are multi systemic effects. Most common clinical signs include weight loss despite increased appetite, hyperactivity, intermittent gastrointestinal signs [vomiting or diarrhea], palpably enlarged thyroid glands, tachycardia [rapid heart rate] and heart murmur. The most common diagnostic tool is a specific blood test T4 and free T4.
Feline hyperthyroidism does not go into spontaneous remission. It cannot be prevented. Left untreated, your cat will progress to being very thin, have severe metabolic problems, cardiac problems and death.
There are several treatment options. Each treatment option has advantages and disadvantages. There are anti thyroid medications [methimazole] that are given orally two to three times a day. They cannot be stopped, require frequent blood tests and there are adverse reactions. Surgical removal of both thyroid lobes is another option. There is an increased anesthetic risk and after surgery, calcium levels can drop [hypoparathyroidism]. There is a new special diet available [Y/D from Hills by veterinary prescription only] that is low in iodine. Your cat must eat this diet exclusively for it to be effective.
Radioactive iodine [I 131] is the safest, simplest, most effective treatment for feline hyperthyroidism. I 131 is 95% effective [curative]. It is given by a subcutaneous injection, is absorbed into the blood stream and concentrates in the thyroid gland. It does require special licensing and isolation for variable periods of time after treatment.
Consult with your primary care veterinarian to see which treatment option is best for your cat if your cat is diagnosed with feline hyperthyroidism.
Dr. Lesley Phillips