Diagnosis and Treatment

Rebecca Norton, DVM
Dennis Heald, DVM, DACVIM, GCVS – Internal Medicine
Tabby cat lying on bedHyperthyroidism is a common disease of cats 8 years old and older. The thyroid gland sits along the windpipe and has two lobes that produce thyroid hormone which regulates the speed of cell function. Too much thyroid hormone (hyperthyroidism) causes cells to work too fast.Although it is not known why cats develop hyperthyroidism, we know it is usually caused by a benign growth (adenoma) in one or both the thyroid glands. Only about 3% of cats with hyperthyroidism have a cancerous thyroid tumor.

Symptoms include weight loss despite a good appetite, increased activity, restlessness, anxiety, hair loss, increased thirst and urine output. Some become irritable or aggressive, have vomiting or diarrhea, panting, decreased appetite, weakness, or listlessness; the latter are less common and associated with “apathetic hyperthyroidism”. Other organs may be involved resulting in a heart murmur and rarely heart failure. Usually signs develop gradually and may go unnoticed for months before treatment is started.

Most affected cats have clear elevations in their thyroid hormone (T4). Occasionally your veterinarian may need to perform other tests, such as “free T4” or thyroid scan to diagnose hyperthyroidism. Sometimes initial testing does not show a clear diagnosis and retesting months later confirms the diagnosis. The thyroid hormone tests are routinely performed at your family veterinary practice, but a thyroid scan requires specialized equipment found at specialty hospitals such as Gulf Coast Veterinary Specialists.

Routine screening of other labwork is standard practice and includes complete blood count (CBC) to assess red and white blood cells and platelets (clotting cells), chemistry profile to assess for underlying diseases of kidneys, liver, or blood sugar (diabetes), urinalysis to further assess kidney function, x-rays of the chest to assess heart and lungs, and occasionally x-rays or ultrasound of the abdomen to assess for other causes of weight loss, vomiting, or diarrhea.

Untreated hyperthyroidism results in progressively ill cats that become thin, weak, and likely to develop life-threatening problems. Treatment will slow the function of cells, and impaired kidney function can be associated with slowed metabolism. For that reason, treatment trials with a reversible form of treatment is recommended to assess kidney function and determine if permanent treatment is best.

The medication used to control hyperthyroidism, methimazole, comes in oral (pill) or topical formulations. It blocks the production of thyroid hormone and is completely reversible. If after a methimazole trial kidney function is decreased, the dose can be adjusted to control the thyroid and maintain optimum kidney function. Both formulations are administered once or twice daily, depending on the level of control needed. The oral form is sometimes associated with vomiting or decreased appetite. The topical formulation is made by a compounding pharmacy and is recommended for cats experiencing side effects or for those difficult to medicate. Rarely methimazole is associated with liver toxicity or with decreases in blood cells that can cause bleeding, predisposition to infection or anemia causing weakness or even death. Occasionally cats with either formulation develop facial itching. While these side effects are alarming, they are rare, and most cats are given methimazole for at least some time during their illness.

Hill’s food company recently developed a prescription diet (Y/D) to control hyperthyroidism by restricting iodine in the food, which reduces the amount of thyroid hormone released. It is fed exclusively of other foods, including medications with flavoring, and cats must be kept indoors as hunting animals outside will increase iodine in the diet enough to negate effects of the prescription diet. If cats without hyperthyroidism are fed this diet, their meals must be supplemented to increase the iodine level ingested.

Surgery to remove the thyroid nodule(s) results in cure of hyperthyroidism. This option may not be recommended for every patient. Rarely cats develop hypothyroidism (too little thyroid hormone) and need supplementation in the form of medication. Radioactive iodine (I-131) is a treatment that cures hyperthyroidism by destroying abnormal thyroid tissue and is effective without repeated treatments in about 95% of cats. A treatment trial with methimazole is recommended first to assess kidney function. Treatment consists of an injection under the skin and is not associated with any adverse effects. I-131 does make cats radioactive for a period of time; state regulations require they be housed in isolation until their level of radioactivity meets a standard considered safe around people (usually 4-14 days). Rarely cats develop hypothyroidism and need supplementation. This treatment is only offered at certain specialty hospitals due to strict safety regulations. Gulf Coast Veterinary Specialists is an approved facility and performs I-131 treatment routinely.

Hyperthyroidism is a disease that affects older cats. If you suspect your cat has symptoms associated with this disease, contact your family veterinarian for testing. Your vet can talk to you about treatment options.

Originally published in January 2013.