Treatment of hyperthyroidism usually is rewarding and extremely effective. The goal of treatment is to reduce the circulating levels of thyroid hormones. There are three options for treatment of hyperthyroidism: daily medications, surgical removal of thyroid tissue, or destruction of the abnormal thyroid tissue through radioiodine therapy. Of the three, only surgery and radioiodine treatments can result in a cure. Medications only can keep the disease under control while they are administered.
Before pursuing more aggressive treatment, many cats are treated with methimazole to bring the thyroid hormone concentration down into the normal range. Ruling out possible concurrent kidney failure is essential before surgery or permanent destruction by radioiodine, since permanent correction of hyperthyroidism can worsen renal disease.
The most commonly used antithyroid drug prescribed to cats is called methimazole. Most cats will need to be given methimazole twice daily for the best results. The drug is introduced at a low dose and then slowly increased to the most effective level for each individual cat. Mild side effects associated with methimazole include loss of appetite, vomiting, and lethargy.
Rare side effects include scratching to the point of causing lesions on the face and neck, a low platelet count, and bleeding disorders. These complications usually develop within the first few weeks of treatment and may require stopping the drug. Contact the veterinarian immediately if a cat on methimazole experiences any side effects.
Drug therapy typically requires frequent veterinary follow-ups for the first three months. Physical exams generally are scheduled every two to three weeks to assess improvements of hyperthyroidism clinical signs, especially of the heart, side effects of the drug, and improvement in the thyroid hormone concentration. Some owners may opt for long-term methimazole treatment if the cat responds well, or fails to develop side effects or resistance to the drug.
Thyroidectomy is the surgical removal of the thyroid gland or glands. If successful, this procedure will correct the hyperthyroidism permanently, unless thyroid cancer is present, or if only one gland is removed initially and the second gland continues or starts to hyperfunction. Antithyroid drugs sometimes are used preoperatively to reduce anesthetic and surgical risks and to rule out concurrent kidney failure.
Possible post-operative complications, rare if the procedure is performed by an experienced surgeon, include critically low blood-calcium levels due to removal or damage to the parathyroid glands, damage to the recurrent laryngeal nerve resulting in laryngeal paralysis, and permanent hypothyroidism requiring long-term thyroid hormone replacement.
Administered by injection, radioactive iodine treatment usually is successful in curing hyperthyroidism and causes the lowest incidence of side effects. However, it is not widely available because special equipment is required, along with strict adherence to radiation safety regulations. State and local radiation safety laws typically mandate in-hospital isolation for eight to 12 days without owner visitation.
Radioactive iodine destroys only functioning thyroid cells. Dormant thyroid cells that do not take up the radiation are then able to produce normal amounts of thyroid hormone at a later time. Very few cats, less than two to four percent, will require a second treatment or require thyroid hormone replacement therapy.
