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Hyperthyroidism in the Cat
by Jennifer Fry, VMD

Hyperthyroidism is a multi-systemic disorder resulting from excessive circulating thyroid hormone. Thyroid hormone regulates metabolism, and therefore, affects almost every system in the body. Hyperthyroidism is probably the most common endocrine disorder that affects cats over 8 years of age. The average age of affected cats is 12 to 13 years old with less than 5% of cats less than 8 years old.

The most common signs of hyperthyroidism are weight loss despite a good appetite, hyperactivity, increased appetite, increased thirst and urination, vomiting, diarrhea, heat intolerance, unkempt haircoat, increased heart rate, abnormal heart beat and/or heart murmur. A few cats may show signs such as anorexia, depression and weakness.

Increased production of thyroid hormone is associated with an enlarged thyroid gland. In most cases the cause of hyperthyroidism is due to a non-cancerous overgrowth (hyperplasia) of the thyroid gland. Approximately 90% of cats with hyperthyroidism will have a palpable thyroid gland or nodule. The thyroid gland is not felt in the normal cat and many clinically normal cats with an enlarged thyroid will eventually develop clinical hyperthyroidism, and therefore, should be monitored. Thyroid cancer is rare and only accounts for only 1-2% of all cases.

Hyperthyroidism is diagnosed by a blood test to measure the level of thyroid hormone. Other blood tests may be performed to evaluate other body systems. There is a link between hyperthyroidism and elevated liver values, and hyperthyroidism can mask kidney problems. Therefore, careful monitoring of liver and kidney values is essential. Frequent examinations by your veterinarian are also very important so s/he can detect changes in your pet especially any problems with the heart.

If your pet is not treated, s/he will continue to exhibit the signs of hyperthyroidism and eventually become sick and suffer. Spontaneous remission does NOT occur; however, with the proper veterinary treatment your cat can resume a normal life.

There are three treatment options for hyperthyroidism:

1. Medication - Methimazole or Tapazole is the most common treatment. There are very few side effects from this drug and patients are usually controlled with one tablet two times a day. Tapazole blocks the production of thyroid hormone but does not destroy the abnormal thyroid tissue. If the medication is discontinued the signs of hyperthyroidism will recur within 24 to 72 hours. The first few months of treatment require repeated blood tests to assure the proper dosage of medication is being given. After the correct dosage is achieved, yearly blood tests are used for monitoring the condition.
   
   
2. Radioactive Iodine - Radioactive iodine provides a simple, effective and safe treatment for cats with hyperthyroidism. The radioactive iodine concentrates in the hyperplastic thyroid cells and destroys the abnormal tissue. This may not be a treatment option if there is concurrent heart or kidney disease. Treatment involves an extended stay at the hospital (5-10 days) and is quite expensive. A single injection of the radioactive iodine usually restores cats to normal levels of thyroid hormone. Occasionally after treatment, hypothyroidism develops and thyroid supplementation is needed. Repeat blood testing is required after treatment.
   
   
3. Surgery - Thyroidectomy or removal of the thyroid gland is an effective treatment for hyperthyroidism; however, there are potential complications such as nerve damage to the eye or voice box, low blood levels of calcium and damage to the parathyroid (the gland that is next to the thyroid gland). Surgery may not be an option for those patients who have heart conditions or other illnesses that prevent them from undergoing anesthesia. After surgery, there is a possibility for recurrence of hyperthyroidism; therefore, thyroid hormone levels should be monitored yearly after surgery.

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