Hyperthyroidism (Feline)

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Feline hyperthyroidism is a condition in which one or both thyroid glands become overactive. If left untreated, the resulting excess of thyroid hormones has harmful effects, including weight loss, hypertension, and damage to the heart, liver and kidneys.

Common name: Feline hyperthyroidism
Scientific name: Feline hyperthyroidism

Diagnosis

Signalment
Feline hyperthyroidism occurs in middle-aged to older cats. Cats fed fish-flavored (particularly salmon) canned cat food or liver and giblets-flavored canned cat food are at increased risk of developing this condition. Siamese and Himalayan cats are at decreased risk of hyperthyroidism compared with other breeds of cats.

Incidence/prevalenceFeline hyperthyroidism is the most common hormonal disorder of cats.

Geographic distribution
There is no geographic distribution for feline hyperthyroidism.

Clinical signs (primary, most to least frequent, scientific term, synonyms)
Weight loss is the most common sign of feline hyperthyroidism. Polyphagia (increased appetite) is frequently noted. Polydipsia (increased thirst) may also occur.

Irritability or restlessness can be seen. Tachycardia (rapid heart rate) may be noticed by the cat's owner, who may feel the cat's heart beating more quickly than usual.

Clinical signs (secondary, most to least frequent, scientific term, synonyms)
Diarrhea may be seen in some hyperthyroid cats. Vomiting or regurgitation can occur, particularly in cats that are eating more rapidly than usual.

Causes (scientific, common term)
Adenomas (benign growths) on one or both thyroid glands are the most common cause of feline hyperthyroidism. More rarely, an adenocarcinoma (malignant tumor) of the thyroid gland may occur. The underlying cause of these growths is not currently known.

Organ systems affected (most to least affected)
Heart, Vascular system: hypertension (high blood pressure) is common. Kidneys, Liver, Hematopoietic system.

Diagnostic tests
Blood tests are used to diagnose feline hyperthyroidism. Measurement of the serum total T4 level is the most reliable test. Measurement of the serum free T4 level may be performed, but falsely positive results can be misleading. A rapid heart rate, high blood pressure and palpable enlargement of one or both thyroid glands are supportive of the diagnosis.

Differential Diagnosis
Inflammatory bowel disease is a common condition that causes similar clinical signs. Intestinal lymphoma is common in cats and also causes similar signs.

Overview

Feline hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It occurs most frequently in middle-aged and elderly cats.

Cats have two thyroid glands located in the neck. Feline hyperthyroidism is caused by the development of a growth on one or both thyroid glands. Most commonly, the growth is of a benign type called an adenoma; more rarely, a malignant tumor of the thyroid gland can occur. Affected glands produce an excess of thyroid hormones.

Elevated thyroid hormone levels cause the heart to beat more rapidly and can result in heart damage and failure. Hypertension (high blood pressure) often develops. Hyperthyroidism can adversely affect the kidneys and may cause elevated liver enzyme levels.

The most common signs of feline hyperthyroidism are weight loss and/or an increased appetite. Affected cats may also become restless or irritable. Increased thirst, diarrhea or vomiting may also be seen.

Treatment

Home Care
A cat exhibiting signs typical of hyperthyroidism should see a veterinarian promptly for examination and testing.

Professional Care
Veterinarians diagnose feline hyperthyroidism through blood tests and palpation of an enlarged thyroid gland. The most reliable test is measurement of a thyroid hormone called the total T4. Cats who have another illness may have decreased levels of thyroid hormones, and this can make the diagnosis of hyperthyroidism more difficult. In this situation, it is most important to diagnose the disease that is depressing the hormone level. In cases where feline hyperthyroidism is suspected but total T4 levels are normal, a test called a free T4 level may be performed, but this test is unreliable, since falsely elevated values may occur with non-thyroidal illnesses. Intestinal diseases may cause similar symptoms to hyperthyroidism and should be suspected in cats that are losing weight and do not have elevated thyroid hormone levels.

Action
Feline hyperthyroidism can be treated with medication, radioactive iodine therapy or surgery. The most commonly used medication is called methimazole (Tapazole¬Æ). Use of this drug requires close monitoring of the cat's kidney function, blood cell counts and liver enzymes because serious side effects can occur. Medication does not cure the disease but will decrease the hormone levels.

Radioactive iodine therapy (RAI) involves an injection of a radioactive isotope. Iodine is used because it is taken up by the thyroid gland. Research has shown that hyperthyroid cats receiving this treatment have the longest lifespan.

Since treatment of hyperthyroidism may affect kidney function, it is recommended that cats be treated with medication before receiving RAI, to ensure that kidney function will remain adequate when thyroid hormone levels normalize.

Surgery to remove one or both thyroid glands may be performed. As with RAI, cats should first be treated with medication to ensure that kidney function will remain adequate. Additionally, normalization of thyroid hormone levels may decrease the risk of anesthetic complications.

Outcome
Cats with feline hyperthyroidism that are promptly diagnosed and treated have a good prognosis. Prognosis is best in cats that receive treatment with medication followed by radioactive iodine therapy.

References/Additional Readings
Milner, Rowan J., et al. Survival Times for Cats with Hyperthyroidism Treated with Iodine 131, Methimazole, or Both: 167 cases (1996 to 2003). In: JAVMA, 2006: 228:559-563.
Martin, Krista M., et al. Evaluation of Dietary and Environmental Risk Factors for Hyperthyroidism in Cats. In: JAVMA, 2000; 217:853-856.

Author
Louise Murray, DVM
DACVIM

Editor
Steven Hansen, DVM, MS, MBA
DABVT, DABT