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Hyperthyroidism & Hypothyroidism





Hyperthyroidism is the most common glandular disorder in cats. It is most frequently caused by an excessive concentration of circulating thyroxine—a thyroid hormone better known as T4—in the bloodstream. The thyroid gland is one of the most important glands in the body. It is located in the neck near the trachea (windpipe) and has two lobes, one on each side of the trachea. This gland is controlled by the pituitary gland, which is located at the base of the brain. The thyroid gland regulates the body’s metabolic rate. If the thyroid is overactive (hyperthyroidism) the body’s metabolism is elevated. If it is underactive (hypothyroidism), the metabolism slows down.


Weight loss and increased appetite are among the most common clinical signs of this condition. Weight loss is seen in 95 to 98 percent of hyperthyroid cats, and a hearty appetite in 67 to 81 percent. Excessive thirst, increased urination, hyperactivity, unkempt appearance, panting, diarrhea and increased shedding have also been reported. Vomiting is seen in about 50 percent of affected cats. Clinical signs are a result of the effect of increased T4 levels on various organ systems. Hyperthyroidism can occur in any breed of cat, male or female, but occurs almost exclusively in older animals. Less than 6 percent of cases are younger than 10 years of age; the average age at onset is between 12 and 13 years.


Because several common diseases of older cats—diabetes, inflammatory bowel disease, intestinal cancer and chronic kidney failure—share some of the clinical signs of hyperthyroidism, a battery of tests is in order. A CBC, chemistry panel and urinalysis alone will not diagnose hyperthyroidism, but they can certainly rule out diabetes and kidney failure. Hyperthyroid cats may have normal findings on the CBC and urinalysis, but the chemistry panel often shows elevation of several liver enzymes. In the vast majority of cases, a definitive diagnosis of hyperthyroidism is based on a simple blood test that shows elevated T4 levels in the bloodstream. Unfortunately, between 2 percent and 10 percent of cats with hyperthyroidism will have normal T4 levels. One possible explanation for this is that in mild cases, T4 levels can fluctuate in and out of the normal range. Another is that concurrent illness will suppress elevated T4 levels, lowering them into the normal or high-normal range and fooling the veterinarian into thinking that the cat’s thyroid status is normal. Because these are geriatric cats, concurrent illness is fairly common, and diagnosis of hyperthyroidism in these cats can be tricky.






Hypothyroidism is a disease caused by a decreased amount of hormones produced by the thyroid gland (under-active), diagnosed by a laboratory test. The thyroid gland is a paired gland located in the throat on either side of the trachea (windpipe).  Treatment of the hypothyroid individual is a simple process of hormone replacement (in pill form) at home. Thyroxine, a thyroid hormone replacement compound, is given based on weight. Generally, once diagnosis is confirmed, this replacement is required for life. Periodically, it will be necessary to retest these individuals to assure that proper levels of replacement are being given. Treatment and control of the hypothyroid individual generally provides very rewarding results for both the patient and owner.


Hypothyroidism is usually caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy. The former disease is the most common cause of hypothyroidism and is thought to be an immune-mediated disease. This means that the immune system decides that the thyroid is abnormal or foreign and attacks it. It is unclear why this occurs. In idiopathic thyroid gland atrophy normal thyroid tissue is replaced by fat tissue. This condition is also poorly understood. These two causes of hypothyroidism account for more than 95% of the cases. The other five percent are due to rare diseases, including cancer of the thyroid gland.


When the metabolic rate slows down, virtually every organ in the body is affected. Most dogs with hypothyroidism have one or more of the following symptoms: weight gain without an increase of appetite, lethargy and lack of desire to exercise, cold intolerance, dry or dull haircoat with excessive shedding and flaking, increased dark pigmentation in the skin, increased susceptibility and occurrence of skin and ear infections, failure to re-grow hair after clipping or shaving, and high blood cholesterol.


The most common screening test is a Total Thyroxin (TT4) level. This is a measurement of the main thyroid hormone in a blood sample. If it is low to below normal and clinical signs are present, this is suggestive of hypothyroidism. Definitive diagnosis is then made by performing a Free T4 by Equilibrium Dialysis (Free T4 by ED). If this test is low, then your dog has hypothyroidism. Some pets will have a low TT4 and normal Free T4 by ED. These dogs do not have hypothyroidism. Additional tests may be necessary based on your pet’s condition. Hypothyroidism is treatable but not curable. It is treated with oral administration of thyroid replacement hormone. This drug must be given for the rest of the dog's life. There is a standard dose that is used initially based on the dog's weight. After one month of treatment, further testing is done to verify that the thyroid hormone levels are normal. The dose will need to be further adjusted and maintained by performing TT4 levels every six months on your pet. Close communication with your veterinarian is necessary in order to ensure that your dog is neither over nor underdosed.