Hypothyroidism is a disorder of the thyroid gland. This gland is responsible for producing and secreting thyroid hormone (thyroxine), which affects nearly all body systems. Thyroxine is the hormone that is primarily responsible for regulating the metabolic rate of many different tissues in the body. In hypothyroidism, not enough thyroxine is produced, which causes the metabolism of these tissues to slow down.
Hypothyroidism is a clinical disease that occurs as a result of decreased blood levels of serum thyroid hormones.
Hypothyroidism is very rare in the cat. It arises most often following treatment of hyperthyroidism in the cat. Hyperthyroidism (overactive thyroid gland) is the most common thyroid disorder of cats.
Causes of Hypothyroidism in Cats
What To Watch For
A deficiency of thyroid hormone affects the metabolic function of many organ systems. As a result, the clinical signs are usually variable, non-specific, and slow to develop. Although there is no one symptom that is diagnostic, several combined signs may make your veterinarian more suspicious. Symptoms may include:
Diagnosis of Hypothyroidism in Cats
Hypothyroidism is not always a simple, straightforward disease to diagnose. Various tests are available to diagnose the condition and a combination of tests may be required. Proper diagnosis also includes a thorough history, documentation of clinical signs, a thorough physical examination, and diagnostic tests to assess various organ functions, including thyroid function. A diagnostic work-up may include the following:
Treatment of Hypothyroidism in Cats
Optimal treatment requires a combination of home and professional veterinary care. At home it is important to administer all medication exactly as prescribed by your veterinarian. With appropriate therapy, most of the clinical alterations associated with hypothyroidism improve within four to six weeks.
Most cats tolerate thyroid supplementation very well, however, over dosage is associated with return of the signs of hyperthyroidism. Watch the cat closely for signs of hyperactivity, increased vocalization (meowing, howling), restlessness, weight loss and diarrhea, and report these signs to your veterinarian promptly.
It is important to follow-up with regularly scheduled visits to your veterinarian so that both your cat’s clinical signs and thyroid concentration in the blood can be monitored. Generally, the first follow-up examination is within four weeks after the start of therapy. T4 levels are often measured six to eight hours after the pill is given. Adjustments in the dosage of medication are then recommended depending upon the results of these tests. Additional recheck visits are then scheduled based upon the test results, changes in clinical signs, and any alterations in the medication schedule.
Prevention of Hypothyroidism in Cats
Careful monitoring during the treatment of hyperthyroidism can often prevent hypothyroidism in the cat. If removal of both thyroid glands can be avoided, then normal output of thyroxine should continue. Periodic measurement of T4 during methimazole therapy helps to prevent over treatment with this drug. Careful dosing of radioactive iodine during radiation therapy of hyperthyroidism is also helpful in avoiding the development of hypothyroidism.