The thyroid gland is a 2-lobed gland in the neck. It produces 2 iodine-containing hormones, T3 and T4, which affect many processes in the body. In general, the thyroid hormones regulate metabolic rate, or the speed at which body processes run. Too little hormone causes body processes to be sluggish. Too much causes them to run too fast.
Thyroid hormones act on many different cellular processes. Some of their actions occur within minutes to hours, while others take several hours or longer. Thyroid hormones in normal quantities work along with other hormones, such as growth hormone and insulin, to build tissues. However, when they are secreted in excess, they can contribute to the breakdown of proteins and tissues.
In hypothyroidism, decreased levels of thyroid hormones result in a slower metabolic rate. In cats, the most common reason for hypothyroidism is surgical removal or destruction (for example, by radioiodine or antithyroid drugs) of the thyroid gland as a treatment for hyperthyroidism. Although naturally occurring hypothyroidism is extremely rare in cats, when it does occur, it appears to be due to a disorder in the thyroid gland itself (rather than in the pituitary gland, as is seen in some other animals).
Because a deficiency of thyroid hormone affects the function of all organ systems, signs vary. In cats with mild disease, signs are mild or absent. In cats with advanced disease, signs include lethargy, dullness, hair loss, low body temperature, decreased appetite, and occasionally decreased heart rate. Obesity may develop, especially in cats with hypothyroidism caused by destruction of the gland stemming from treatment for hyperthyroidism. In cats that are born with hypothyroidism (or that develop it at a young age), signs include dwarfing, severe lethargy, mental dullness, constipation, lack of appetite, and decreased heart rate.
Accurately diagnosing hypothyroidism requires close evaluation of signs and various laboratory tests, including demonstration of low blood concentrations of thyroid hormones (especially T4) in combination with high levels of thyroid-stimulating hormone.
Hypothyroidism is treated using replacement with synthetic thyroid hormone. The success of treatment can be measured by the amount of improvement in signs. Thyroid hormone concentrations are also monitored to determine whether the dosage of thyroid hormone needs adjustment. Once the dose has been stabilized, thyroid hormone levels are usually checked once or twice a year. Treatment is generally lifelong.
Hyperthyroidism is caused by excess of the thyroid hormones, T3 and T4. It is most likely to be seen in middle-aged to old cats. A hormone-producing, benign thyroid tumor is the most common cause of feline hyperthyroidism. Both thyroid lobes are typically enlarged and may be felt by your veterinarian.
Signs of hyperthyroidism reflect an increased metabolic rate. The most common signs include weight loss, excessive appetite, hyperexcitability, increased thirst and urination, vomiting, diarrhea, and increased fecal volume. Cardiovascular signs include increased heart rate, murmurs, shortness of breath, an enlarged heart, and congestive heart failure. Rarely, hyperthyroid cats have signs such as reduced appetite, lethargy, and depression.
Diagnosis is based on the history, signs, and physical examination, and confirmed by a blood test to measure the thyroid hormone level.
Cats with hyperthyroidism can be treated by radioactive iodine treatment, surgical removal of the thyroid gland, longterm administration of an antithyroid drug, or lifelong nutritional therapy (with a prescription diet that restricts iodine). Radioactive iodine is usually recommended; it is simple, effective, and safe. The radioactive iodine concentrates within the thyroid tumor, where it irradiates and destroys the overactive thyroid tissue without affecting other tissues.
Surgically removing the thyroid gland is also effective. If the tumor affects only one side of the gland, only that half is removed and treatment with synthetic thyroid hormone usually is not needed. If the tumor affects both sides of the gland, the entire gland must be removed and treatment with synthetic thyroid hormone will be needed after surgery. The main complication is that the parathyroid glands, which sit on either side of the thyroid gland, can be injured or also removed during surgery. In this case, treatment with calcium and vitamin D will also be needed.
Daily treatment with methimazole, an antithyroid drug, blocks the production of thyroid hormone. Because most adverse effects associated with methimazole treatment develop during the first 3 months, complete blood counts and serum thyroid hormone measurements are checked frequently (every 2 to 4 weeks) during that time. The dosage of methimazole is adjusted to maintain circulating thyroid hormone levels within the normal range. After this initial period, levels of serum thyroid hormone are usually measured every 3 to 6 months to monitor response to treatment and the need for further dosage adjustments. Additional medications may also be necessary to treat complications of hyperthyroidism.
A prescription diet is available for cats that cannot receive other therapies. The food is deficient in iodine, which is an essential component of T4 and T3. Dietary therapy appears to be most effective in cats with mild hyperthyroidism and may not work for cats with severe disease. Cats must eat only this food for the rest of their lives; other cat foods, table foods, or treats will make the therapy ineffective.