Hyperthyroidism in Cats
The thyroid gland, located inside the lower neck, plays an important role in regulating the body’s rate of metabolism. Hyperthyroidism is a disorder characterized by enlargement of the thyroid gland, followed by subsequent overproduction of thyroid hormone and an increase in a cat’s metabolic rate. This condition is relatively common in senior cats. In the majority of cases, the thyroid gland enlargement is a benign tumor; less than 2% involve a thyroid gland malignancy. Many organs are affected by hyperthyroidism, including the heart. The disease stimulates the heart to pump faster and more forcefully, and eventually the heart enlarges to meet these demands. The increased output of blood from the heart may lead to high blood pressure and, in some cases, heart failure. The liver is also susceptible to damage by overproduction of thyroid hormone, as are the kidneys and gastrointestinal tract.
Cause of hyperthyroidism
The cause of hyperthyroidism in cats remains unknown. Environment and diet have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific risk factors and mechanisms are unknown. No breed is known to be at increased risk.
The typical cat with hyperthyroidism is middle-aged or older, though the condition has been reported in younger adult cats as well. The most consistent finding among cats with this disorder is weight loss secondary to the increased rate of metabolism. Many cats compensate for this loss by eating more than usual; some hyperthyroid cats may develop a ravenous appetite. Despite the increased intake of food, most cats gradually lose weight. The weight loss can be so gradual that some owners do not notice it until their cat has lost 10–20% of his body weight. Affected cats may also drink more water and pass larger volumes of urine than they did prior to developing this condition. Other physical symptoms include periodic vomiting, soft stool or diarrhea, and an unkempt hair coat. Some cats develop anorexia as the disease progresses. Behavioral symptoms include elimination outside the litter box, increased irritability, increased activity levels, increased vocalization and other changes.
Two secondary complications of this disease can be significant. They include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the pressure exerted on the arteries by the increased output of blood from the heart. Untreated hypertension, as in people, may lead to strokes. Severe hypertension may also cause retinal hemorrhage or detachment and sudden blindness. Thyrotoxic cardiomyopathy occurs when the heart muscle stretches and thickens due to excess production of thyroid hormone; as a result, the heart pumps less efficiently. Both hypertension and thyrotoxic cardiomyopathy are reversible with appropriate treatment, especially if the conditions are diagnosed at an early stage.
In its early states, hyperthyroidism may mask declining kidney function because the hypertension that often accompanies hyperthyroidism increases blood flow to the kidneys. When a cat with underlying kidney insufficiency is treated for hyperthyroidism/hypertension, renal blood flow returns to normal and the kidney insufficiency may become more apparent. Conversely, hyperthyroidism left untreated can have a deleterious effect on the kidneys as well.
In most instances, diagnosis of this disease is relatively straightforward:
- Enlargement of one or both thyroid lobes is one of the first changes to occur. The thyroid glands are too small to be easily palpated (felt) during a physical examination of a non-hyperthyroid cat. If the glands can be palpated, the cat likely has hyperthyroidism.
- The first blood test performed to diagnose hyperthyroidism measures the level of thyroxine (T4), one of the thyroid hormones. A T4 level elevated beyond the normal reference range usually confirms the diagnosis.
- If a cat is suspected of having hyperthyroidism (i.e., has a T4 level within the upper range of normal limits but not above normal, along with symptoms consistent with an overactive thyroid), a second test, called a Free T4, may be performed on the same blood sample to help confirm the diagnosis. Alternatively, repeating the T4 level in 4-6 weeks may show a T4 level above normal. Additional testing (thyroid stimulating hormone, thyroid scan) may be necessary for some cats.
Options for treatment of hyperthyroidism in cats include radioactive iodine therapy, medical management (most often oral medication), dietary management, and surgical removal of the affected thyroid gland(s). Because the large majority of cats with hyperthyroidism do not have cancerous tumor growth, treatment can be completed in most cats using relatively low dosages of radioiodine. (Cats with cancerous tumors will require much higher dosages of radioiodine).
Radioiodine treatment is considered the gold standard treatment for hyperthyroidism in most cats, especially in younger cats and those without advanced kidney disease. It is curative 97% of the time with a single treatment, and in most cases, treatment with radioiodine eliminates the need for long-term oral thyroid medication. It is the most effective and least invasive way to destroy all of the abnormal thyroid tissue. Veterinary clinics must be licensed to administer radiation therapy. After treatment, patients must be hospitalized until their level of radioactivity has declined to a level that is low enough to allow release back into their homes (usually 2-5 days at our facility).
Administration of an oral drug, usually Felimazole (methimazole), can interfere with thyroid hormone production and thus control the effects of the overactive thyroid gland. Medical management is not curative.
The large majority of cats tolerate this drug quite well. However, some cats may react adversely to the drug. Side effects, including vomiting, lethargy, anorexia, itchiness and anemia, may begin to occur as late as several weeks after the drug is started. In some cats, use of alternate drugs, or alternate routes of medication may be tolerated better than methimazole.
Since medical management of hyperthyroidism is not curative, it does not destroy the abnormal thyroid tissue. Thus, the drug must be given for the remainder of a cat’s life. Oral medication must be administered daily (usually twice per day), and the dosage often needs to be adjusted periodically to maintain normal thyroid hormone levels.
Additionally, while on methmiazole, because hyperthyroidism is a progressive disease, the thyroid tumor will continue to grow since the drug blunts production of thyroid hormone by the tumor, but does nothing to prevent growth of the tumor. Longer-term medical management is an acceptable option as long as the drug is well tolerated by the cat and the cat’s thyroid levels are adequately controlled with regular administration. However, because the thyroid tumor continues to grow with the medical management treatment option, with time (usually several years), it is not uncommon for thyroid levels to become less able to be well controlled with medical management, and more difficult to easily address with radioiodine because the disease is now more advanced and/or may have transformed into a malignant tumor.
Bloodwork is generally done every three to six months to monitor thyroid, liver, and kidney values but may need to be done more frequently if the thyroid levels are not well regulated or if there are concurrent health issues.
Dietary management: In late 2011, a prescription veterinary diet was introduced for the treatment of hyperthyroidism in cats. Although the exact cause of hyperthyroidism has not been identified, a diet low in iodine has shown potential as a means of reducing elevated thyroid levels. Cats being treated with diet alone must eat the diet exclusively.
Surgery: The fourth option is surgical removal of the affected thyroid lobe(s). Because hyperthyroid cats are usually older and because heart and metabolic abnormalities are associated with hyperthyroidism, a degree of risk is involved. Surgery is a more invasive procedure, and postoperative complications may arise. If ectopic thyroid tissue (tissue in abnormal locations) is not detected and therefore not removed during surgery, hyperthyroidism may redevelop. Depending on the patient and the patient’s owner, surgery may be an appropriate treatment option (and it is considered a cure, as is radioiodine), but in most cases, radioiodine is preferable.
The outcome following treatment of hyperthyroidism is usually positive, and most cats have a good chance of returning to an excellent state of health for many years. It is one of the few diseases in older cats that we can easily treat, and even cure (with radioiodine in particular), though there are pros and cons of all available treatment options.
No preventive measures are known, but middle-aged and geriatric cats should have a complete physical examination by a veterinarian every 6–12 months.