Treatment Options

Radioiodine Therapy

Cat Radioiodine Therapy

This is the safest and most effective therapy for feline hyperthyroidism. Because normal thyroid tissue is spared, cats do not typically require thyroid hormone supplementation. General anesthesia is not required for therapy, eliminating a major risk factor for the geriatric cat.

Since starting Feline Hyperthyroid Treatment Center, P.S. in 1992, we have achieved a 98.5% “cure” after a single injection and a 99.8% “cure” after a second injection. Less than 0.25% of the cats treated required supplementation for hypothyroidism.

Side effects in cats are so rare that we always look for something else as a cause if cats are ill after treatment. We must remember we are dealing with a geriatric population that may have any number of potential underlying problems. So, if your cat is ill after returning home, this is atypical. Always see your veterinarian and tell them to look for something else. If nothing else can be found, then we can consider an unusual reaction to the treatment. Our concern is when owners assume their cat is reacting normally if they’re not eating, active, etc. and don’t see their veterinarian. Many of these old cats are too fragile to not eat or drink for a couple days.

Medical Management

Medical management would seem to be the simplest and cheapest option available, but this often proves to be untrue since the medication can have severe side effects and must be used for the cat’s entire life. Thyroid supplements do not cure the disease or kill the tumor cells causing the problem. Side effects of methimazole (Felimazole®) are common and include anorexia, vomiting, lethargy, excoriations, bleeding, hepatopathy, thrombocytopenia, agranulocystosis, leukopenia, eosinophilia, lymphocytosis, positive ANA, and positive direct antiglobulin test.

In our experience, cats rarely return to normal on methimazole (Felimazole®). Most cats seem to improve but never quite reestablish their prehyperthyroid state. Owners must be aware of this potential risk when selecting medication as their medical choice.

Another misconception is that medical treatment costs less. Medications can cost $1 - $2 per day or approximately $350 to $700 per year. Monitoring side effects and dosage adjustments mandates that cats be re-examined and have blood tests repeated every 3-6 months. Assuming a more practical 6-month interval and an average cost of $85 - $100 for an examination, general diagnostic and T4, the annual cost incurred would be $170 - $200. Some cats require more frequent follow-ups and incur greater costs. A conservative annual cost for medical management is approximately $600 - $900, year after year.

When considering actual costs, the inconvenience of twice daily pilling a cat for life, adverse effects of the medications, progressive cardiomyopathy that we can’t resolve with medicine, and considering we are merely suppressing the disease and not curing it, medical management is rarely the best option for treating hyperthyroidism. Medical management may be the best choice in cats with concurrent life threatening diseases (e.g. cancer, severe renal disease) and whose life expectancy is short, hence the cost of therapy may not be justified for a few months of life.

Medical management for any length of time can make therapy with radioiodine more difficult by allowing the abnormal thyroid tissue to continue to grow which then requires higher doses of radioiodine and therefore a longer hospital stay for the cat.


Surgical removal (thyroidectomy) of the diseased lobes of the thyroid gland has been a popular treatment. Most veterinarians suggest removal of both lobes due to the high probability (70 - 90%) that both are affected. When only one lobe is removed, the disease typically recurs in the other lobe within 1-2 years.

Surgery presents significant risks to the patient. First, and foremost, surgery often requires prolonged anesthesia of a geriatric patient who typically has compromised renal functions and varying degrees of heart disease. Surgery typically only involves thyroid tissue in the cervical area, any remaining ectopic (extra) tissue can perpetuate the hyperthyroidism.

Salvaging functional parathyroid tissue to maintain normal calcium and phosphorus levels is critical, but in some cases impossible. Albeit there is a wide range of success depending on surgical technique and skill. At best, cats with bilateral thyroidectomy typically require lifetime thyroid hormone supplementation (pills).

Thyroidectomy is the preferred treatment in some cats with severe decompensated heart failure where immediate resolution of the hyperthyroid state is required. These cats have heart disease so severe that it cannot be managed medically and cardiologists may advise an immediate thyroidectomy.

Nutritional Management (y/d)

Hill’s, a leader in the pet food industry, introduced Prescription Diet y/d in 2011. It is an iodine-deficient food that reduces thyroid hormone production by eliminating a necessary component of the hormone. Controversy exists regarding the total elimination of iodine and the existing protein limitations inherent to y/d.

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