Many organs are affected by this disease, like the heart and the kidneys. The heart is stimulated to pump faster and more forcefully; eventually, the heart enlarges to meet these increase demands for blood flow. The increased pumping pressure leads to a greater output of blood and high blood pressure. About 80% of cats with hyperthyroidism have high blood pressure. This increased blood flow also affects how the kidneys function. The disease often masks kidney failure.

No individual breed is known to be at increased risk, although the Siamese appears to have a 10-fold lower risk of developing hyperthyroidism than other breeds.

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 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 problems can be reversible if the problems are caught early and appropriate treatment is started.

Several tests are needed before adopting any form of treatment. These tests are needed to evaluate the overall health of the cat and predict the chances for complications. Such tests include blood work, urinalysis, x-rays, and sometimes an ECG and cardiac ultrasound may need to be performed.

Cats may have renal (kidney) disease that is masked by the hyperthyroidism. For this reason, it is important to perform a trial of Tapazole then recheck the blood work to look for hidden kidney disease before choosing a permanent treatment like surgery or iodine therapy.

1. Surgery : Surgical removal of the affected thyroid lobes is also very effective. Because hyperthyroid cats are usually over 8 years of age, there is a degree of risk involved. However, if the cat is otherwise healthy, the risk need not be considered significant. Because both thyroid lobes are removed, most cats need to be on long term thyroid supplementation after surgery. No adverse side affects have been seen with thyroid supplements.

If surgery is the treatment method chosen, the cat is usually treated with an anti-thyroid medication for several weeks prior to the operation. During that time, the ravenous appetite should subside and the cat will probably gain weight. Some cats also have a very fast heart rate and high blood pressure; these problems can be managed with medication before surgery. After one to two weeks, another T4 level is measured.

The cat is generally not hospitalized following surgery as most cats are more comfortable at home and more likely to eat. Eating habits should be normal after returning home. 

The most common and serious complication is loss of parathyroid function. Since the parathyroid gland is so small, its artery is also very tiny. If this vessel is not preserved during surgery, the parathyroid gland will quit functioning for several days or permanently. In some cases, parathyroid function will be disrupted just by the handling involved in surgery. In either case, if both parathyroid glands are not functioning the result will be a dramatic fall in blood calcium levels (controlled by the parathyroid gland), seizures, and, in some cases, death.

2. Radioactive iodine : The most effective way to destroy all of the abnormal tissue is with radioactive iodine therapy. The treatment usually requires only one injection and anesthesia is not necessary. It causes no damage to normal thyroid tissue or to the nearby parathyroid gland. This requires 3 to 7 days of hospitalization at a veterinary clinic licensed to administer radiation therapy. The cat needs to be otherwise healthy since it will be in isolation for a period of time. 

3. Oral medication : Administration of an oral drug, Felimazole (Methimazole) can control the effects of the overactive thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side effects may begin as late as months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Some cats become allergic to this medication and show symptoms of facial itching. Methimazole does not destroy the abnormal thyroid tissue, but rather ties up the excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat’s life. Periodic blood tests must be done to keep the dosage regulated. This type of treatment is appropriate for the cat that is a poor surgical and radioactive iodine candidate due to other health problems. As stated above, this drug may also be used for a few weeks to stabilize the cat that is at increased surgical risk because of cardiac complications.

4) Diet : Small studies have shown the reduction of iodine in a cat’s diet lowers thyroid hormone in the cat’s body. An iodine reduced prescription diet food is available. This diet is still very new and at this point a last resort when other treatments are not possible.