Definition of Hyperthyroidism in Cats
Hyperthyroidism is the condition caused by excessive functional activity of the thyroid gland, resulting in excessive secretion of the thyroid hormones, thyroxine and triiodothyronine. It is the most common endocrine disease in domestic cats, and is extremely rare in dogs.
How Hyperthyroidism Affects Cats
Cats with this disease show a number of classic signs, including increased thirst and corresponding increased urination, weight loss despite increased appetite and food consumption, restlessness, hyperactivity, possible aggression, respiratory difficulty, elevated respiratory and heart rates and cardiac arrhythmias. They also tend to have an unkempt appearance and poor haircoat. A small percentage of hyperthyroid cats show weakness, lethargy, depression and anorexia. Affected cats typically have bilaterally enlarged thyroid gland lobes, although occasionally only one lobe is affected.
This disease predominantly affects older cats. There is no sex predilection. Purebred cats are much less likely to be affected than are mixed breeds.
Causes of Hyperthyroidism in Cats
Feline hyperthyroidism is the clinical condition caused by continual excessive secretion of thyroid hormones. There is no known genetic predisposition to developing this disease. Usually, the increased production of thyroid hormones is due to either a benign but functional thyroid tumor or autonomous enlargement of the thyroid gland. Most cases of hyperthyroidism in cats are caused by the increase in size of either one or both lobes of the thyroid gland, which continue to function and produce abnormally large amounts of thyroid hormones. This enlargement is caused by excessive cell division within the thyroid tissues; the reason for this hyperplasia is not well understood. Cancer is a much less common cause of hyperthyroidism in cats, but is more so in dogs. Thyroid hormones are responsible for moderating vital bodily functions, such as metabolic rate, protein synthesis and overall cell health. Excessive levels of these hormones disrupt these functions and cause the symptoms associated with hyperthyroidism.
Preventing Feline Hyperthyroidism
There is no realistic way to “prevent” hyperthyroidism in cats.
The prognosis for hyperthyroid cats with uncomplicated disease is excellent in the short-term and good in the long-term, as long as there is consistent owner compliance with medical treatment protocols. Radioiodine treatment and thyroidectomy are extremely successful in most cases. Cats with underlying renal disease have a more guarded prognosis, since both diseases are systemic and progressive. Kidney failure is the most common cause of death in hyperthyroid cats.