Hyperthyroidism in Cats

Hyperthyroidism is an excess of circulation thyroid hormones. It results in a high metabolic state, which causes changes in many different body organs.
In most cats, the cause of this disorder is unknown. It may be related to environmental, nutritional, or other factors.
Clinical Signs:
Most cats are middle-aged (average age at onset, 12-13 years). Signs are often gradual in onset, become more severe over time, and include;
+ Weight loss, muscle wasting, decreased ability to jump onto objects.
+ Increased appetite.
+ Vomiting
+ Increased thirst and urination
+ Nervousness, hyperactivity, increased vocalization.
Physical examination may reveal a palpably enlarged thyroid gland or glands, thin body condition, heart murmur or irregular heartbeat or both, high heart rate, excessive shedding and matting, or poor quality hair coat. Muscle weakness, abnormal gait, retinal hemorrhages or detached retinas, apathy, and dehydration may also be noted.
Diagnostic Test:
Hyperthyroidism is diagnosed by measurement of a thyroid hormone (T4) in the blood.
+ T4 is increased in 95% of affected cats.
+ T4 may be falsely low or normal in cats with other illnesses, so the diagnosis can be difficult to make.
Routine laboratory tests may reveal alterations in white blood cell count, elevated liver tests, and low potassium concentration. Laboratory tests may also show the presence of kidney disease, which is common in older cats.
Treatment Options:
Several options exist, including medical therapy, surgery, and radiation therapy with radioactive iodine. The most common medication used is methimazole, which comes in pill form or can be compounded into a paste to apply to the ear. It is given once or twice daily and can be given in the presence of kidney disease.
Follow-up Care:
Routine laboratory and T4 assays are done after many of the treatments and are repeated periodically for cats on medical therapy.
Most clinical signs improve with treatment, but some heart and eye changes can be permanent. Cats with kidney disease and hyperthyroidism tend to be more difficult to manage.