By Anthony Carr, DVM, Dipl. ACVIM

Hyperthyroidism is the most common endocrinopathy in adult cats. Usually, the disease is caused by adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are relatively rare.

In 70 percent of the cases, the changes of the thyroid gland are bilateral. You can see many clinical signs. For example, weight loss is present in about 90 percent of cases and polyphagia in approximately 50 percent of cases. In addition, other signs can be observed such as hyperactivity, polyuria / polydipsia, tachycardia, high blood pressure, vomiting, diarrhea, panting, generalized weakness, among others. In rare cases, apathetic hyperthyroidism with weakness, inappetence and marked lethargy is observed.

The diagnosis, in most cases, is easy since most cats will have high total T4 values. In some cases, especially in cats with another underlying disease or in older cats, total T4 may even be in the normal reference range.

In this situation, additional tests may be useful, especially that of free T4 with dialysis procedure.

The objective of the treatment of hyperthyroidism is to reach a euthyroid state. This condition can be achieved with medications, radioactive iodine therapy, ethanol injections or thyroidectomy. The treatment chosen will depend on several factors, including cost, other concurrent diseases, especially renal failure, and the advantages and disadvantages of different therapies.

Adverse side effects of medical therapy may make this treatment impossible in some patients; therefore, you should look for other options. In almost all patients, you should try to perform medical therapy before resorting to more permanent therapies.

Hyperthyroidism causes an increase in the Glomerular Filtration Rate (GFR), which can hide the severity of underlying kidney problems. Medical treatment will help identify if kidney problems develop with euthyroidism in a reversible manner. If renal function deteriorates significantly, the permanent solution of hyperthyroidism with surgery or I-131 therapy may be contraindicated.

The medical treatment of hyperthyroidism includes the management of concurrent diseases, especially those of the cardiovascular system. The state of hyperthyroidism, especially if chronic, can cause a variety of cardiovascular abnormalities, including hypertension. Tachycardia is common with or without arrhythmias. It can be observed murmurs, gallop rhythm, and occasionally signs of congestive heart failure. Hypertension is present in approximately 10 and 20 percent of cats recently diagnosed with hyperthyroidism (personal data).

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