Veterinary Help

Hyperthyroidism in cats is caused by overactivity of the thyroid gland most often seen in middle aged to older cats. This is due to an overgrowth of functional thyroid tissue. In general the thyroid can be viewed as the body’s metabolism control and with increased activity the body goes into overdrive.Clinical signs may be evident in various body systems but tend to progress slowly. Affected cats will usually have evidence of weight loss despite a normal to increased appetite. They often drink and urinate more then normal. They may be more active, restless or irritable than was once normal.

Gastrointestinal signs include vomiting and/or diarrhea. The excess thyroid hormone may cause an increased heart rate and can lead to changes in the heart muscle itself. In some cases this leads to heart failure. These changes may be reversible once the hyperthyroidism is controlled but not in all cases. These cats can also have decreased ability to deal with stress leading to panting and open mouth breathing in stressful situations.Underlying kidney disease is an especially important consideration in cats with hyperthyroidism. Since older cats are typically affected with both disease processes they may occur together though it is not thought that there is a cause and effect relationship. The hyperthyroid state can both harm and help the kidneys. The harm comes if high blood pressure is caused by hyperthyroidism. This in turn leads to the progression of kidney disease. The help comes from increased filtration of blood through the kidneys helping maintain their function. Treating hyperthyroid cats with underlying kidney disease is a balancing act and these patients need to be monitored closely

Methimazole is a drug used to manage hyperthyroidism. It does not cure the disease, but interferes with thyroid hormone production, controlling thyroid hormone levels as long as it is given. Upon discontinuing this medication thyroid levels will again rise. Because of this it is often the first choice treatment in cats with kidney disease when the decision has been made to treat them. Methimazole is not without risks. The most common side effects are vomiting and anorexia. These gastrointestinal signs may resolve with a lower dose or symptomatic treatment while continuing on the methimazole. More serious effects include facial excoriations, blood cell count problems and exacerbation of underlying kidney disease. For these reasons, during the initiation of therapy it is important to regularly monitor not only blood levels of the thyroid hormone but also blood cell counts and kidney values. Regular physical exams are essential as well.

Radioactive iodine therapy is considered the treatment of choice for many hyperthyroid cats. It involves the injection of radioactive iodine (I131). Since the thyroid is the only body tissue that concentrates iodine it is specifically targeted allowing the radiation to destroy the hyperactive thyroid tissue. This procedure is done at specialized facilities equipped to handle the radiation and require the patient spend several days while the radioactivity dissipates. This method does offer a cure.