Feline Hyperthyroidism

Hyperthyroidism is the most common glandular disorder in cats. It is most frequently caused by an excessive concentration of circulating thyroxine-a thyroid hormone better known as T~4-in the bloodstream.Co-incident with the introduction of polybrominated diphenyl ethers (PBDEs) into household materials nearly 30 years ago, feline hyperthyroidism (FH) has increased dramatically

What Are the Symptoms of Hyperthyroidism?

Weight loss and increased appetite are among the most common clinical signs of this condition. Weight loss is seen in 95 to 98 percent of hyperthyroid cats, and a hearty appetite in 67 to 81 percent. Excessive thirst, increased urination, hyperactivity, unkempt appearance, panting, diarrhea and increased shedding have also been reported. Vomiting is seen in about 50 percent of affected cats. Clinical signs are a result of the effect of increased T₄ levels on various organ systems.

What Breeds/Ages Are Prone to Hyperthyroidism?

Hyperthyroidism can occur in any breed of cat, male or female, but occurs almost exclusively in older animals. Less than 6 percent of cases are younger than 10 years of age; the average age at onset is between 12 and 13 years.

How Is Hyperthyroidism Diagnosed?

Because several common diseases of older cats-diabetes, inflammatory bowel disease, intestinal cancer and chronic kidney failure-share some of the clinical signs of hyperthyroidism, a battery of tests is in order. A CBC, chemistry panel and urinalysis alone will not diagnose hyperthyroidism, but they can certainly rule out diabetes and kidney failure. Hyperthyroid cats may have normal findings on the CBC and urinalysis, but the chemistry panel often shows elevation of several liver enzymes.

In the vast majority of cases, a definitive diagnosis of hyperthyroidism is based on a simple blood test that shows elevated T₄ levels in the bloodstream. Unfortunately, between 2 percent and 10 percent of cats with hyperthyroidism will have normal T_4 levels. One possible explanation for this is that in mild cases, T₄levels can fluctuate in and out of the normal range. Another is that concurrent illness will suppress elevated T_4 levels, lowering them into the normal or high-normal range and fooling the veterinarian into thinking that the cat’s thyroid status is normal. Because these are geriatric cats, concurrent illness is fairly common, and diagnosis of hyperthyroidism in these cats can be tricky.