Hyperthyroidism is a multisystemic metabolic disorder that occurs as a result of excessively high concentrations of thyroid hormones in the bloodstream. It is the most common glandular disorder in cats. Untreated hyperthyroidism results in increased cardiac output, and increased blood flow to the kidneys. Although the disease is treatable, in recent years it has become apparent that previously undetected kidney disease may suddenly be unmasked, and that known kidney disease may worsen in some cats after correction of their hyperthyroidism.

The disease is caused by excessive secretion of thyroid hormones by hyperplastic (overly large), usually benign thyroid glands. Although extensively studied, it is still not known why some cats develop hyperthyroidism while others do not.

There is no breed or sex predisposition for the disorder, but it occurs almost exclusively in older cats. Less than 6 percent of cases are younger than 10 years old. The average age of onset is 12 to 13 years.

The impact of the disease can be significant, as hyperthyroidism can affect many body systems, especially the heart.

What to Watch For

Weight loss

Ravenous appetite

Excessive drinking and urinating

Hyperactivity (restlessness, excessive vocalization)

Intermittent vomiting

Intermittent diarrhea

Panting

Diagnosis

Complete blood count

Chemistry panel

Urinalysis

Thyroid hormone level

Radiographs

Scintigraphy

Treatment

Antithyroid medication (tapazole, carbimazole, ipodate)

Possible cardiac medication

Surgical removal of the thyroid glands

Radioactive iodine therapy

Consequences involving the kidneys can be minimized or prevented in hyperthyroid cats by assessing kidney function before undertaking therapy to treat the hyperthyroidism. Cats are then given oral medication to bring the thyroid hormone levels back into the normal range slowly, temporarily controlling the condition. Kidney function is then re-evaluated. If kidney function remains stable, a more permanent method of therapy (surgery, radioactive iodine) can be undertaken. If kidney function worsens, the veterinarian is faced with the dilemma of balancing the two disorders so that the most severe clinical signs of either disorder are kept to a minimum.

Home Care and Prevention

Consistently administer any prescribed anti-thyroid and/or cardiac medication if necessary. Treat for kidney failure (subcutaneous fluids, prescription diets, phosphate binders, gastric protectants, vitamin D supplements) if warranted and prescribed.

It is still not understood why some cats develop hyperthyroidism while others do not. Thus, there are no measures that can be taken to prevent the development of hyperthyroidism.