Hyperthyroidism in Cats


Feline hyperthyroidism is a disorder that affects multiple systems in a cat's body. It is a common disease, and it can have devastating effects on a cat's health if it is left undiagnosed and untreated.

History of Feline Hyperthyroidism

Scientists first described this condition in the late 1970's, and the diagnosis of hyperthyroidism has steadily increased since that time.

Veterinarians are more aware of the disease and therefore diagnose it more often, but there is also suspicion that there are more cases today than 30 years ago.

Although plenty of time and resources have been devoted to studying feline hyperthyroidism, its cause is still elusive. Some believe that an imbalance of iodine in commercial cat foods or the exposure to environmental materials such as hydrocarbons (phenols, phthalates, etc.) potentially predispose humans and animals to thyroid diseases. You can learn more in these articles: "Flame Retardants and Hyperthyroidism in Cats," "Fishy Food and Feline Hyperthyroidism," and "Homemade Diets for Cats."

Others think that it is possible that hyperthyroidism in cats is caused by circulating immune system factors. Many hypotheses exist, but none have proven to be the single accepted reason why cats become affected by this disease.

What Is Hyperthyroidism in Cats?

Hyperthyroidism is the most common endocrine disease affecting middle-aged and older cats. Male and female cats are affected equally, as are all feline breeds.

Most cats have hyperthyroidism due to benign growths in their thyroid gland that increase the size of the gland (multinodular adenomatous hyperplasia) and its function. This results in an increase in the concentration of the thyroid gland hormones T3 and T4 in the cat's circulation. Although malignant thyroid tumors have been reported, they are rare and account for only 1-2 % of cats with hyperthyroidism.

Signs of Feline Hyperthyroidism

All of the clinical signs associated with hyperthyroidism are due to the increased amounts of T3 and T4, which generally have a stimulating effect (think cats on large amounts of coffee). Although there are many ways a cat with this disease can appear, most become very thin, and owners usually note a markedly increased appetite. The spike in appetite is due to the increased need for energy because of an accelerated metabolic rate.

Sadly, these cats cannot eat enough calories to sustain themselves. There have been reports of desperate cats attacking other pets in the family and even owners at the dinner table in an attempt to obtain more food.

Diseased cats can also be restless and nervous, vocal, display stumbling or weakness, have respiratory disease or heart arrhythmias, develop an unkempt and matted coat, experience vomiting and diarrhea, and exhibit excessive thirst and urination. Some cats will have an obviously enlarged thyroid gland (palpable goiter), but due to the hair coat and location of the gland lobes (next to the windpipe along the neck), this is something that owners rarely notice.

Over time, hyperthyroidism causes heart disease and high blood pressure in cats, which can both have devastating health effects.

Diagnosis of Hyperthyroid Disease in Cats

The diagnosis of feline hyperthyroidism is made by evaluating the cat's history and clinical signs, and it is confirmed with laboratory tests. Although other diseases can have signs similar to hyperthyroidism (diabetes, kidney disease, cancer, and chronic intestinal disease), some blood tests will help the veterinarian determine which disease or diseases are present. An increase in thyroid hormone levels in the blood of a cat that exhibits the clinical signs of hyperthyroidism is diagnostic for the disease.

Occasionally, cats with early, mild hyperthyroidism may have normal blood levels of T3 and T4. If the veterinarian still suspects the disease, a second blood test should be taken a few weeks later, due to the fluctuation of the hormones in and out of the normal range in the early stage of the disease. Other tests exist to deny or confirm the diagnosis of hyperthyroidism (radioiodine uptake and diagnostic scanning, T3 suppression testing, and TRH response testing). These are less commonly used.

A free T4 by equilibrium dialysis is a specific blood test that can often help diagnose hyperthyroidism when the T4 is still in the normal range, so your veterinarian may also recommend that test.

The veterinarian will screen for general health of the cat, especially for the presence of kidney disease, heart disease, and high blood pressure.

Treatment of Feline Hyperthyroidism

Treatment of hyperthyroidism in cats usually has good results and is aimed at reducing functioning thyroid hormone levels. There are 3 ways this goal may be accomplished. Your veterinarian is best suited to help you decide which treatment is best for your cat based on a number of factors.

Prognosis for Hyperthyroidism in Cats

Whatever method of treatment the health care team decides to pursue for a hyperthyroid cat, the outcome is usually highly rewarding. Most cats can be brought back to health, and even serious consequences of the disease such as heart dysfunction can often be reversed with proper treatment. Concurrent kidney disease can pose special challenges, but your veterinarian will advise you of the preferred course of action.

If you suspect that your cat might be suffering from this condition, consultation with the family veterinarian to develop a plan for diagnosis and treatment is necessary.

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