The International Bengal Breeders' Association October 2001 News Letter

Feline Hyperthyroidism

Hyperthyroidism is a common disease of middle aged and older cats.  It is the result of an over production of the thyroid hormones commonly referred to as T3 (tri-iodothyronine) and T4 (thyroxine) by an over active or enlarged thyroid gland.  Environmental factors such as nutrition and environment may stimulate the thyroid to produce an excessive amount of thyroid hormones.

Common symptoms include weight loss, increased appetite, increased thirst and urination, hyperactivity, diarrhea, vomiting and muscle weakness.  The coat may look dull an unkempt.  High levels of T3 and T4 can lead to heart disease, which can be manifested by increased heart rate, arrhythmias, difficulty breathing and heart murmurs.

Hyperthyroidism is usually diagnosed by a complete chemistry panel and thyroid hormone levels.  During physical examination, the thyroid will be palpated to determine if one or both lobes of the thyroid are enlarged.  In a majority of cats diagnosed with hyperthyroidism, both lobes are usually enlarged.

There are three basic types of treatment for hyperthyroidism: (1) radioactive iodine (2) surgical removal of the thyroid gland (3) antithyroid medications such as Tapazole.  Each type of treatment has advantages and disadvantages.

Radioactive iodine selectively destroys functioning thyroid tissue, and is considered a curative form of treatment.  Locating a veterinary hospital which performs this type if treatment may be difficult.  Radioactive iodine has a half-life of 8 days, necessitating a prolonged hospital stay of around 7 to 10 days.  During this 7 to 10 day period, the radioactive isotope is potentially hazardous to humans and other animals.  The current average cost is between $850-$1000.

Surgical removal of the enlarged thyroid lobe or lobes is also curative.  Cats with hyperthyroidism are at increased anesthetic risk usually due to the effect that the disease has had on the cat's metabolism and heart.  Post operative complications can occur.  The most common post operative complication is hypocalcaemia, which occurs in response to injury or inadvertent removal of the parathyroid gland or glands.  This complication usually manifests itself in the first 72 hours following surgery, symptoms include weakness, muscle tremors, tetany (muscle spasms) and convulsions.  This can be treated by  calcium supplements.  When both thyroid glands are removed, a daily thyroid supplement is required for the rest of the cat's life.  If only one lobe is diseased and removed, the remaining healthy lobe can usually maintain proper hormone balance without additional treatment or supplements.  The average cost of the surgery is between $500 -$700.

Antithyroid drugs alleviate the signs and symptoms of the disease by decreasing the production of thyroid hormones.  Antithyroid drugs are not curative.   Adverse reactions include loss of appetite, vomiting, lethargy, skin rashes, facial edema (swelling) and itching.  Antithyroid drugs are often administered one to three times daily.  In addition, frequent blood testing to monitor thyroid levels in needed.  In some cases antithyroid drugs become ineffective.  The cost of the treatment and blood monitoring varies from geographical location, averaging $250-$600 yearly.

© October 2001 by The International Bengal Breeders' Association (TIBBA)
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