Brightwood Animal Hospital

9640 Old Johnnycake Ridge Rd
Mentor, OH 44060


Feline Hyperthyroidism

What is Feline Hyperthyroidism?

Feline hyperthyroidism is a systemic disorder caused by excessive thyroid hormone. Thyroid hormone regulates metabolism, and therefore, affects almost every system in the body. Hyperthyroidism is the most common endocrine disorder diagnosed in cats over 8 years of age. The average age of onset is 12 to 13 years.

What are the signs of Feline Hyperthyroidism?

The signs of hyperthyroidism include weight loss, hyperactivity, increased appetite, increased thirst and urination, vomiting, diarrhea, heat intolerance, poor hair coat, increased heart rate, abnormal heart beat and/or heart murmur. Advanced cases may show signs such as anorexia, depression and weakness. These are general signs and only raise the suspicion of a thyroid disorder.

What causes Feline Hyperthyroidism?

Increased production of thyroid hormone results from an enlarged thyroid gland. In most cases the enlargement is due to a non-cancerous overgrowth (hyperplasia) of the thyroid gland. Many times your veterinarian may even be able to feel the enlargement of the gland. The thyroid gland is difficult to feel in a normal cat. Many clinically normal cats with an enlarged thyroid will eventually develop signs of hyperthyroidism. Thyroid cancer is rare and only accounts for only 1-2% of all cases.

How is Feline Hyperthyroidism diagnosed?

If after examining your cat the veterinarian finds the signs of hyperthyroidism a blood test to measure the level of thyroid hormone can be done. Other blood tests may be performed to evaluate other body systems. There is a link between hyperthyroidism, liver, kidney and heart problems. Therefore, careful monitoring of liver and kidney function is essential. Frequent examinations by your veterinarian are the best way to detect changes in your pet especially any problems with the heart.

If your cat is not treated, it will continue to exhibit the signs of hyperthyroidism and eventually become sick and suffer. The disease will not resolve on its own; however, with the proper veterinary treatment your cat can resume a normal life.

How do Veterinarians treat Feline Hyperthyroidism?

There are four treatment options for hyperthyroidism:

Medication - Methimazole or Tapazole is the most common treatment. There are 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, your veterinarian may recommend regular blood tests to monitor the condition.

Diet - Hill's Y/D prescription diet is a severely Iodine restricted diet that effectively lowers thyroid hormone levels in cats that are fed only that diet.  Other cats in the household that are not hyperthyroid can also eat Y/D if they are given regular cat food once a week to keep their Iodine levels up.  

Radioactive Iodine - Radioactive iodine provides a simple, effective and safe treatment for cats with hyperthyroidism. The radioactive iodine concentrates in the thyroid and destroys the abnormal tissue. This may not be the best treatment option if there is also heart or kidney disease. Treatment involves an extended stay at the hospital. A single dose of the radioactive iodine usually restores normal thyroid hormone levels. Occasionally after treatment, hypothyroidism develops and thyroid supplementation is needed. Repeat blood testing is required after treatment. This treatment is only available at hospitals licensed to use radioactive medications.

Surgery - Thyroidectomy or removal of the thyroid gland is an effective treatment for hyperthyroidism. 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. With the advent of radioactive iodine treatments, surgery is less likely to be recommended.

by William M. Fraser, D.V.M.

Brightwood Animal Hospital serves Mentor, Concord, Painesville and the surrounding communities.

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