Hyperthyroidism is a generalized, high metabolic state caused by oversecretion of thyroid hormones by the thyroid glands. It is a common disease in adult cats. The thyroid glands are a pair of small, soft glands that lie deep in the tissues on the underside of the neck. In a cat, each normal thyroid gland is about the size of a cooked rice grain. In most cases of hyperthyroidism, the condition is caused by overfunctioning benign tumors (nodules) in the thyroid glands. Malignant thyroid tumors are rare, causing only 1–2% of cases of hyperthyroidism in cats. Hyperthyroidism is rare in dogs and is usually caused by thyroid tumors or oversupplementation with thyroid hormones used for treating low thyroid levels.


Since thyroid hormone is a major controller of the body’s metabolic rate, a common symptom of hyperthyroid cats is that they typically lose weight despite having an increased— sometimes ravenous—appetite. Vomiting and diarrhea may also occur, due to the overeating and the changes in metabolism of the intestine. Some cats drink more water and urinate larger volumes than normal, but many other diseases may cause this symptom, too. Hyperactivity is a common sign, and other behavioral changes may occur, such as aggressiveness. Hyperthyroid cats often give the impression of being surprisingly active and energetic for their age, and unfortunately this effect of excess thyroid hormone is outweighed by its negative impact on the heart, the muscles, and the skin, all of which are overburdened by hyperthyroidism. It is important to realize that although the increased energy level of a hyperthyroid cat is appealing, the negative effects that come with it are harmful, and treating hyperthyroidism often causes a cat to “slow down,” showing a decrease in energy level that is more appropriate and is compatible with more normal, long-lasting function of the heart, muscles, skin, and other organs. During an examination, your veterinarian may detect a rapid heart rate or a heart murmur when listening to the chest with a stethoscope because the increased metabolic rate present in hyperthyroidism affects the heart. Your veterinarian may also be able to feel the enlarged thyroid gland simply by palpation (feeling the lower neck with the fingertips).


Most cases of hyperthyroidism can be diagnosed by routine testing for thyroid hormone (T4), which is a simple blood test that specifically measures the circulating level of thyroid hormone. Rarely, some cats suspected of having hyperthyroidism will have normal or equivocal (“grey-zone”) T4 levels on blood testing, and additional tests may be recommended to determine whether or not hyperthyroidism is truly present. Chest x-rays or ultrasound examinations may be recommended to evaluate the heart effects of hyperthyroidism


Untreated hyperthyroidism gradually produces increasingly serious symptoms, with severely affected cats becoming emaciated, ravenously hungry, and hyperactive, extremely irritable, or aggressive. Cats that are successfully treated by any of the three methods described below can return to normal, and the long-term outlook for a normal lifespan is good.


Hyperthyroidism can be managed one of three ways: with daily antithyroid medication, with surgical removal of the thyroid gland, or with radioactive iodine treatment. From a medical standpoint, the best of the three is radioactive iodine treatment, but it may not be easily accessible or available.


  • During methimazole treatment, if you notice any of the side effects listed in the treatment section. SIGNS TO WATCH FOR Indications of uncontrolled hyperthyroidism: • Excessive appetite with weight loss. • Vomiting of food or diarrhea. • Hyperactivity or inactivity. • Drinking excessive amounts of water or urinating larger volumes than normal.


  • Cats on methimazole therapy need examination (for the veterinarian to listen to the heart with the stethoscope and check your cat’s weight) and laboratory testing (blood samples) every few weeks during the first few months to monitor for side effects, and periodically thereafter based on symptoms

About CVC

Chappelle Veterinary Clinic is a full service Veterinary Clinic which offers a wide range of veterinary care for cats and dogs, including routine exams, vaccinations, preventative care, diagnostics, surgery, spay and neuter, wellness services, dental cleaning, emergency and more.

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Monday 8:00 AM – 6:00 PM
Tuesday 12:00 PM – 9:00 PM
Wednesday 8:00 AM – 9:00 PM
Thursday 8:00 AM – 6:00 PM
Friday 8:00 AM – 6:00 PM
Saturday 8:00 AM – 3:00 PM
Sunday Closed

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14128 28 Avenue SWEdmonton, AB T6W 3Y9

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