The last Fully Vetted column discussed hypothyroidism, which is a common endocrine disorder of dogs. Today’s column will discuss hyperthyroidism, which occurs commonly in cats, but very rarely affects dogs.

Hyperthyroidism occurs when the thyroid glands become enlarged and produce too much thyroid hormone. The most common cause of thyroid gland enlargement in cats is a non-cancerous tumor called an adenoma. Some cases of hyperthyroidism may be due to a type of malignant tumor called an adenocarcinoma, although these are uncommon.

Thyroid hormones regulate the metabolic rate of most of the cells in the body. With hyperthyroidism, the presence of excess thyroid hormone causes the cells to work harder and faster than they should. This increased metabolic rate negatively affects many body systems.

Thyroid hormones trigger an increase in both the rate and contraction strength of the heart. This causes thickening of the heart muscle, which can lead to heart failure.

Hyperthyroidism also leads to high blood pressure, or hypertension. Hypertension causes damage to many organs, including the kidneys, eyes, and brain. 

A variety of clinical signs may be associated with hyperthyroidism. These include weight loss, hyperactivity, excessive vocalization, increased appetite, and increased thirst and urination. Vomiting, diarrhea, or a greasy haircoat may also be noted.

Diagnosis of hyperthyroidism requires a blood test. Since hyperthyroidism affects multiple body systems, it is important that your veterinarian thoroughly evaluates the overall health of your pet. This may require additional diagnostic tests such as bloodwork, blood pressure measurement, and a urinalysis.

Several options are available to treat hyperthyroidism. An oral medication, which is usually given twice daily, is usually an effective method of treatment. This medication can also be compounded into an ointment that is applied topically. Treatment must be continued for life, and thyroid levels must be evaluated periodically.

Surgical removal of the thyroid glands via a thyroidectomy is another treatment option.  This method requires general anesthesia, and is more risky in patients who are older or have concurrent kidney or heart disease.

Radioactive iodine treatment is offered by many teaching and specialty hospitals. This method of treatment involves injecting radioactive iodine into the bloodstream. The substance is taken up by the thyroid gland, where it destroys abnormal thyroid tissue. This method of treatment is very effective, but it is usually expensive, and requires several days of hospitalization. 

A prescription diet is also available to treat hyperthyroidism. The diet works because it is free of iodine. Since the thyroid glands require iodine in order to function, a lack of iodine in the diet causes the thyroid glands to decrease production of thyroid hormones. If dietary therapy is chosen, it is extremely important that the patient receive absolutely nothing else to eat. Even a small treat could contain enough iodine to negate the positive effects of the diet.

Hyperthyroidism can severely affect a cat’s health. The good news is that it is treatable, and in many cases, secondary disease processes, such as heart and kidney disease, are also reversible with treatment.