Hyperthyroidism is one of the most common diseases of the feline endocrine system. Estimated to affect approximately one in every 300 cats, hyperthyroidism is most commonly caused by non-malignant changes of the thyroid gland, most often adenoma (a non-malignant form of neoplasia) or hyperplasia (an abnormal increase in the number of cells).
Symptoms of Feline Hyperthyroidism
Hyperthyroidism most frequently affects middle-aged to older cats, with the median age at diagnosis being 13 years. Increased levels of thyroid hormones circulating throughout the feline body affect almost all organ systems, making hyperthyroidism a multisystemic disease.
The most typical signs of feline hyperthyroidism are:
- weight loss
- increased appetite
- increased thirst
- increased urine production
- restlessness
- poor hair coat
Other organ systems can also be affected by hyperthyroidism. Cardiovascular abnormalities seen include a systolic heart murmur, congestive heart failure and even dyspnea (difficulty breathing) secondary to congestive heart failure. Gastrointestinal signs such as occasional or intermittent vomiting may also be seen.
Hypertension (high blood pressure) has also been associated with hyperthyroidism in cats. There is disagreement about how often hyperthyroid cats suffer from hyperthyroidism, with ranges listed from 25% to 87% of hyperthyroid cats suffering from secondary hypertension. As a result, blood pressure should be monitored for cats with hyperthyroidism as a matter of course.
Hypertension, particularly when persistent, can result in central nervous system vascular disease. Though rare, hyperthyroid cats have been reported to suffer from central nervous system signs such as head tilt, paresis or paralysis resulting from vascular accidents associated with hypertension secondary to hyperthyroidism. Ocular disease can also be seen as a result of hypertension.
Treatment of Hyperthyroid Cats
Feline hyperthyroidism can be treated in a number of different ways. Medical management is possible. Methimazole is the drug of choice for managing hyperthyroid cats. Methimazole is often effective in controlling the symptoms of hyperthyroidism. However, medical management is not curative and methimazole must be continued for the lifetime of the cat unless another treatment option is elected.
Hyperthyroidism can also be treated surgically, by removing the diseased thyroid gland (thyroidectomy). While this surgery is curative and does not require special facilities (unlike radiotherapy), there are risks inherent with the surgery.
- Patients are frequently senior or geriatric cats with potential heart disease which make them less than ideal surgical candidates.
- Hyperthyroidism can recur if diseased thyroid gland tissue remains post-surgically.
- Neurological disease is possible post-surgically, including a change in voice and Horner's syndrome.
- Hypothyroidism may occur post-surgically and require permanent hormone replacement therapy.
- Hypocalcemia may occur post-surgically due to the absence of the parathyroid gland which is sometimes removed or damaged during the surgery due to its proximity to the thyroid gland. Hypocalcemia can be life-threatening if not treated promptly. Calcium supplementation may be necessary temporarily or permanently after the surgery.
Radiotherapy involves treatment with iodine 131, a radioactive isotope of iodine, which destroys the abnormal thyroid tissue. This treatment is curative and is generally considered safer than surgical removal of the thyroid glands.
Hyperthyroidism and Hypertension in Cats
Hyperthyroid cats which have persistent hypertension which is classified as moderate to severe (a systolic blood pressure greater than 160-170 mm Hg on two or more occasions) or those which have evidence of ocular disease or central nervous system deficits secondary to hypertension may require antihypertensive therapy. Neurological deficits secondary to hypertension may resolve with proper treatment, though regression may take 3-4 weeks or longer.
Drugs commonly used to treat hypertension in hyperthyroid cats include:
- calcium-channel blockers, such as amlodopine
- ACE inhibitors, such as enalapril or benazepril
- beta-blockers, such as atenolol
Medications may need to be combined to successfully treat hypertension secondary to feline hyperthyroidism. Often, single-agent therapy is not sufficient.
Beta-blockers can mask the effects of hyperthyroidism because of their effects on beta-receptors. Therefore, atenolol may actually provide an advantage in treating hypertension in hyperthyroid cats. It should be used cautiously with calcium-channel blockers, however.
ACE inhibitors appear to be the least effective treatment for hypertension in cats.
Hyperthyroidism in Cats
Feline hyperthyroidism is a commonly diagnosed disease in cats which can be treated medically, surgically or with radiotherapy. If present, hypertension may also need to treated to successfully manage hyperthyroid cats.
Sources:
Feldman EC, Nelson RW; "Feline hyperthyroidism (thyrotoxicosis)"; Canine and Feline Endocrinology and Reproduction, 3rd ed.; 2004.
American College of Veterinary Internal Medicine; "Guidelines for the identification, evaluation and management of systemic hypertension in dogs and cats"; Journal of Veterinary Internal Medicine; 3: 542-558, 2007.
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