Hyperthyroidism

Feline hyperthyroidism is a disorder that affects multiple systems in the cat’s body.

Hyperthyroidism is a very common disease and can have devastating effects if left undiagnosed and untreated.

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 thirty years ago.

Although plenty of time and resources have been devoted to studying this disease, little is still known about the cause of feline hyperthyroidism. Some believe that an increase in the amount 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. The special lining chemical in canned foods is an example. Others think that it is possible that hyperthyroidism in cats is caused by circulating immune system factors. Many theories exist, but none have proven to be the single accepted reason why cats can become affected with this disease.

Hyperthyroidism is the most common endocrine disease affecting middle-aged and older cats. Male and female cats are affected equally, as are all breeds. Most cats develop benign growths in their glands that increase the size of the gland (multinodular adenomatous hyperplasia). This causes an increase in the concentration of the thyroid gland hormones T3 and T4 in the circulation. Although malignant thyroid tumors have been reported, they are rare and account for only 1-2 % of cats with hyperthyroidism.

Signs

All of the clinical signs associated with hyperthyroidism are due to the increased amounts of these hormones, 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 increased appetite is due to the increased need for energy being developed owing to the cat’s 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, show stumbling or weakness, have a respiratory disease or heart arrhythmias, an unkempt and matted coat, and exhibit increased thirst and urination. Some cats will have an obviously enlarged thyroid gland as well (palpable goiter), but due to the haircoat and location of the gland lobes (next to the windpipe along the neck) this is something that owners rarely notice.

Diagnosis

The diagnosis of feline hyperthyroidism is made on the bases of the history, clinical signs, and confirmation with laboratory tests. A thorough physical examination should be performed by a veterinarian. Although other diseases can have signs similar to hyperthyroidism (diabetes, kidney disease, cancer, and chronic intestinal disease) a blood sample will help the veterinarian determine which disease or diseases are present, as cats with hyperthyroidism may suffer from other concurrent illnesses as well. An increase in the 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.

The veterinarian will also screen for general health of the cat, especially for the presence of kidney disease, heart disease and the blood pressure since many cats with hyperthyroidism have these other problems concurrently.

Treatment

Treatment of hyperthyroidism is usually rewarding and is aimed at reducing functioning thyroid hormone levels. This can be accomplished by stopping production of the thyroid hormones from the thyroid gland or by destroying or removing overactive thyroid tissue by surgery or radiotherapy. Oral medications may help to bind excess hormone, but do not fix the problem. They are used if the cat has concurrent heart or kidney disease and is not stable enough for surgery, or until an appointment for radioiodine therapy can be made.

Complete blood counts and T4 levels should be checked regularly during the first 3 months of therapy with oral medications. It is important to use the lowest possible effective dose. Some cats will have side effects from this drug including vomiting and weakness, low platelet counts, anemia, extreme facial itchiness, and severe liver disease.

Thyroidectomy, or surgical removal of or the thyroid gland can be performed by most feline veterinarians. In the case of a malignant cancer of the gland, removal of invasive tumors often requires a surgical specialist. There are few disadvantages to surgical removal. If the patient is in advanced geriatric years, or in a debilitated condition though, surgery and anesthesia will be a bit more complex. If vessels or nerves, or the parathyroid glands which inhabit the area of the thyroid gland are affected during surgery, side effects will occur. Parathyroid gland parameters are carefully monitored for by the health care team, and prompt treatment instituted if the function is compromised.

Although rare, some cats that undergo thyroidectomy will relapse into hyperthyroidism a few years after the surgical procedure due to re-growth of the nodules of the thyroid lobes. However, surgery has the benefit of usually being curative and if successful, releases the owner from the need to medicate the cat on a daily basis.

Administration of radioiodine is the third option for treatment. This method of treatment has several advantages over the others. It avoids the hassle of daily oral medications and the potential side effects of anti-thyroid drugs,and is without the risks of anesthesia for surgical thyroidectomy. It lacks serious side effects. The major drawbacks of this type of treatment are the cost, the lack of widely available facilities that can perform the procedure, and the need for the cat to be confined to a restricted area of the hospital for a short term. It used to be that for one to three weeks after treatment, the cat would need to be restricted to the area, but newer regimens allow for quick return to the caregiver in many treatment facilities. Most experts recommend avoiding close, prolonged contact with the treated cat or the urine and feces during the first two weeks that they are home.

Whatever method of treatment the health care team decides to pursue, 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 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 an owner suspects that their cat might be suffering from this condition, consultation with the family veterinarian to develop a plan for diagnosis and treatment is needed.