Overview
Contributing Factors
Clinical Signs
Causes
Diagnosis
Treatment Options
Prognosis
Prevention

Overview
back to the top
The thyroid gland is located on the underside of the neck (as it is in people) and plays a very important role in regulating the body’s rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This is a fairly common condition in older cats. Although the thyroid gland enlarges, it is most often a nonmalignant change (benign). Less than 1% of hyperthyroid cases involve a thyroid gland malignancy.

Many organs are affected by the hyperthyroid condition, including the heart. The heart is stimulated to pump faster and more forcefully; eventually, the heart enlarges to meet these increased demands for blood flow. The increased pumping pressure leads to a greater output of blood which may lead to high blood pressure and in some cases heart failure.   The liver is also susceptible to damage by excessive amounts of thyroid hormone, as are the kidneys and gastrointestinal tract.

Contributing Factors
back to the top

Advancing age is the main factor that increases a cat’s risk for hyperthyroidism. Environmental and dietary risk factors have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific factors and mechanisms remain unknown.

No individual breed is known to be at increased risk.

Clinical Signs
back to the top

The typical cat with hyperthyroidism is middle-aged or older. The most consistent finding with this disorder is a loss of weight secondary to the increased rate of metabolism. The cat often compensates for this with an increased appetite. In fact, some hyperthyroid cats may have a ravenous appetite and will eat more than theyever have historically.   Despite the increased intake of food, most cats gradually lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred until the cat has lost 10-20% of his body weight. Affected cats may also drink more water and urinate larger volumes than they did prior to developing this condition.   Additionally, there may be periodic vomiting, soft stool or diarrhea, and the hair coat may be unkempt. In some cats, anorexia develops as the disease progresses.  Other signs include elimination outside the litter box, increased irritability, uncharacteristic ‘kitten-like’ activity levels (‘he acts like a kitten again!’), as well as other behavior changes.

Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called “thyrotoxic cardiomyopathy”. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that strokes may occur, or retinal hemorrhage or detachment will occur and result in sudden blindness. The heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these conditions are reversible with appropriate treatment of the disease, especially if the conditions are diagnosed at an early stage.

Diagnosis
back to the top

In most instances, diagnosis of this disease is relatively straightforward.

• One of the first changes to occur is an increase in size of one or both thyroid lobes. The thyroid glands are too small to be easily palpated (felt) during a physical examination in normal (non-hyperthyroid) cats.  In the hyperthyroid cat,  however, as the thyroid glands enlarge, the veterinarian is often able to feel a ‘thyroid nodule’ when examining the underside of the neck area.
• The first blood test that is performed measures the level of one of the thyroid hormones, called thyroxine (or T4). Usually, the T4 level is elevated above normal limits such that there is no question as to the diagnosis.
• Occasionally, a cat suspected of having hyperthyroidism will have a T4 level within the upper range of normal limits. When this occurs, a second test, called a Free T4, may be performed on the same blood sample to help confirm the diagnosis.
• If neither of these tests is diagnostic, and hyperthyroidism is still suspected, a thyroid scan can be performed at a veterinary referral center.  Alternatively, because T4 levels wax and wane, the T4 test may be repeated 2-3 weeks later.

Treatment Options
back to the top

Because less than 1% of hyperthyroid cats have cancerous growths of the thyroid gland, treatment is usually very successful – and may be successful in cats with cancerous thyroid nodules as well. There are three choices for treatment:  medical management, radioactive iodine treatment, and surgical removal of the affected thyroid gland(s).  Many factors must come into consideration when choosing the best therapy for an individual cat.

• Initial treatment in most cases usually involves medical management, most often oral medication. Administration of an oral drug, usually Felimazole for Cats (methimazole), can blunt thyroid production and thus control the effects of the overactive thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 10%). However, the side effects may begin as late as several weeks after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. If methimazole causes side effects, there are two other medications that can be prescribed instead. Most cats will tolerate at least one of the three medications available to blunt thyroid production. Methimazole does not destroy the abnormal thyroid tissue, but rather ties up the excess thyroid hormone. Therefore, if this is the only treatment utilized, the drug must be given for the remainder of the cat’s life. Long-term medical management is an acceptable option as long as the medication is well tolerated by the cat and the cat’s thyroid levels are adequately controlled with regular administration of the drug. Owners should be aware that this option does require daily (usually twice daily) administration of oral medication for the rest of the cat’s life and that periodic adjustments in the dosage are usually needed to maintain normal thyroid hormone levels. Bloodwork is generally done every three to six months to monitor thyroid, liver and kidney values. This bloodwork may need to be done more frequently if the thyroid levels are not well regulated or if kidney function is deteriorating. (Underlying renal insufficiency or mild chronic renal failure can be worsened when the thyroid hormone is lowered to a normal level because there is subsequently less blood flow to the kidneys.)  In those cats that are intolerant of the Felimazole, or whose owners are unable to pill the cat, other options for drug administration are available (alternate drugs, transdermal gels, chews, liquids, etc.).
• Once the thyroid level is well regulated and we know that the kidney function is stable with treatment of the overactive thyroid, permanent corrective procedures are possible, for those cats in whom long-term medical management is not the best option. The two permanent corrective options are radioactive iodine treatment and surgical thyroidectomy.  These options may be considered with or without a medical management (oral medication) trial first.
• Radioactive iodine. The most effective and least invasive way to destroy all of the abnormal tissue is with radioactive iodine therapy. It is almost always curative, and may be even in the rare cases of malignant thyroid tumors. This requires 2-5 days of hospitalization at a veterinary clinic licensed to administer radiation therapy. In most cases no supplemental thyroid is necessary after radioactive iodine treatment. In younger cats (8-12 years of age, or so), with normal kidney function at the time of the diagnosis of hyperthyroidism, we will often recommend the radioactive iodine without first doing a medication trial.
• Dietary Management  In late 2011, a prescription veterinary diet was introduced for the treatment of hyperthyroidism in cats.   Although an exact cause of hypethyroidism has not been identified, a diet specifically formulated to be reduced in iodine levels has shown potential in reducing elevated thyroid hormone levels in cats.
• Surgery. Surgical removal of the affected thyroid lobe(s) is also performed. Because hyperthyroid cats are usually over 8 years of age and because of the heart and metabolic abnormalities associated with hyperthyroidism, there is a degree of risk involved. The disadvantages of surgery are that it is an invasive procedure and there are potential post-operative complications. Additionally the cat may go on to become hyperthyroid again sometime after surgery if there is ectopic thyroid tissue (tissue in abnormal locations) that isn’t removed during surgery. For these reasons and due to the availability of radioactive iodine therapy now in the Chicago area, we rarely recommend surgery as an option any longer.

As was mentioned above, one potential consequence of treating hyperthyroidism is the unmasking of underlying kidney insufficiency or failure. In some geriatric cats, kidney function declines with age. When hyperthyroidism is also present, at least in its early stages, it may partially compensate for this loss of kidney function. This is because the hypertension that accompanies hyperthyroidism serves to increase blood flow to the kidneys. When the hyperthyroid/hypertensive state is treated, renal blood flow returns to normal and underlying kidney insufficiency becomes more apparent. (It existed prior to thyroid treatment, but was “masked” by the hypertension-induced increased kidney blood flow.) This occurs in only a small number of hyperthyroid cats and is most likely if kidney function tests before hyperthyroid treatment are abnormal. Conversely, if a cat’s hyperthyroid condition is left untreated, this can have a deleterious effect on the kidneys as well. Therefore kidney and thyroid levels are monitored very closely in all geriatric cats, and especially closely in those with abnormalities in both of these organ systems.

Prognosis
back to the top

Many owners of cats with hyperthyroidism are hesitant to have radiation therapy because of their cat’s advanced age. Remember, however, that old age is not a disease. The outcome following both medical management and radiation therapy is usually very positive, and most cats have a very good chance of returning to an excellent state of health for many years.

Prevention
back to the top

There are currently no known preventive measures, but middle-aged and geriatric cats should all receive a complete physical examination by a veterinarian every 6-12 months.