Feline Hyperthyroidism, Part Two: Treatment

Dr. Laci Schaible

There are three ways to treat hyperthyroidism in cats: medically, surgically, and with radiation therapy. Each different treatment modality carries its own pros and cons and the choice between these various options is based on the clinical needs of the pet, costs, and the experience of the clinician and/or the availability of radiation.

Medical Management

Medical management of hyperthyroidism consists of the use of antithyroid drugs. Antithyroid drugs block the formation of the thyroid hormones T3 and T4.The most common antithyroid drug is methimazole.

Side effects associated with the use of methimazole drugs include vomiting, lethargy, anorexia, liver enzyme abnormalities and other blood work abnormalities, among others. Due to the potential negative side effects, blood work should be routinely performed.

The problem with medical management of hyperthyroidism is owner compliance. Most patients require twice daily dosing. Many cats are difficult to orally pill, to say the least, and pet parents become frustrated with trying to medicate their pets. Often it even impacts the feline/pet parent bond.

Compounded methimazole (liquid and transdermal) may offer a solution, but has issues unique to compounded drugs. Compounded pharmacies lack regulatory controls and the consistency/effectiveness of the product may vary significantly.

Surgical Management

Thyroidectomy is the surgical removal of the affected thyroid gland and is an effective treatment for cats with diagnosed hyperthyroidism. However, surgery has several disadvantages. Hyperthyroid cats tend to be older/frailer and are potentially at increased risk for surgical and anesthetic problems. Since many of these cats have secondary cardiac disease, pre-operative treatment with medications is often recommended. If your cat is healthy enough to undergo anesthesia and surgery, this may be an option, but it does carry serious risks.

Several different thyroidectomy techniques have been developed in an attempt to minimize potential post-operative complications associated with bilateral thyroidectomy such as hypocalcemia or recurrence of hyperthyroidism. Damage to, or removal of, all four parathyroid glands during bilateral thyroidectomy causes hypocalcemia, the most common post-operative complication.

Radioactive Iodine

The isotope radioiodine 131I is the treatment of choice for eligible hyperthyroid cats because of its high rate of success (95% cure rate with one treatment) and low rate of morbidity/mortality. Radiation therapy is simple, safe, effective, requires no anesthesia, and the risk of hypoparathyroidism is eliminated. It is administered as a subcutaneous injection, an oral preparation, or an IV injection. However, for safety purposes, the subcutaneous route is the preferred treatment.

Reduction in serum total T4 is usually achieved within days to several weeks, but the treatment is not without its drawbacks. The disadvantages of using radioactive iodine include: expensive cost up front, possibility of a lengthy hospitalization, proximity of a facility that offers the treatment, and the risk of post treatment-kidney failure.

The unsuspecting side effect of all therapies

While hyperthyroidism is often successfully treated or controlled, depending upon the treatment option selected, many felines experience a decline in kidney function following successful treatment. This effect is seen with medical and surgical treatments as well as following radioiodine therapy. In the majority of cats, the decline in renal function is mild and clinically insignificant. However, in a small percentage of cats the decline in kidney function can be dramatic.

To attempt to avoid this complication, it is generally recommended that all hyperthyroid cats be treated with low dose methimazole therapy and have laboratory values rechecked in one to two weeks. Long-term definitive therapy should only be undertaken in those cats with stable kidney function following initial medical management.

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