Yes, although it is very uncommon (less than 5% of treated cats). These recurrences usually develop 3 years or more after the hyperthyroidism was first treated. Therefore, such relapses may stem from a new thyroid tumor, rather than the first tumor re-growing.

Our research studies have shown that a single dose of radioiodine will cure 95% to 98% of hyperthyroid cats. Those cats that are not completely cured after initial treatment will still improve clinically because most of these cats have some decrease in their circulating thyroid hormone concentration. If your cat is in the small minority that is not cured within 3 to 6 months after treatment, we generally recommend retreatment with radioiodine as the best treatment method. Virtually all cats that remain hyperthyroid after the first treatment are cured by the second treatment.

Other facilities may boast that they can cure a higher percentage of cats, but they do so at the expense of making many more of their cats hypothyroid (or underactive thyroid). The bottom line — it is easy to give a large dose of radioiodine to all hyperthyroid cats and lower the T4 value, but it is much more difficult to avoid creating a state of hypothyroidism. This requires the calculation of tumor volume, which we do with thyroid scintigraphy (scanning).

We recommend monitoring at 1, 3, 6, and 12 months after treatment, and every 6-12 months thereafter. At each of those visits, a serum thyroid panel (including serum T4 and TSH concentration) and serum chemistry analysis (at least serum creatinine) should be repeated to monitor thyroid and kidney status.

If you or your veterinarian email us a copy of these test results, along with an update of your cat’s clinical status at time of testing, we would be happy to help with test interpretation, comments, and recommendations.

After the 12-month recheck, additional monitoring visits are recommended at least once yearly.

The long-term prognosis for hyperthyroid cats treated with radioiodine is excellent. Many owners are hesitant to treat older cats with radioiodine. But remember, old age is not a disease. Almost all cats (including cats of all ages) return to normal health and live a completely normal life after radioiodine therapy.

The prevalence of hypothyroidism varies according to treatment center and is primarily related to the dose of radioiodine that has been administered. If your cat’s thyroid tumor is small and a large dose of radioiodine is administered, this will certainly cure the hyperthyroidism. However, such an overdose will also destroy much of any normal thyroid tissue and lead to hypothyroidism, an underactive thyroid condition.

We do our best to give the “perfect” dose of radioiodine to each hyperthyroid cat we treat, individualizing the dose given to each cat. We do this by determining the amount of thyroid tumor tissue present in your cat (by thyroid scintigraphy). We raise or lower the dose, as needed, to give just enough radioiodine to destroy your cat’s thyroid tumor, but we try to give the smallest dose possible in order to preserve any normal thyroid tissue.

About 20%  of cats treated develop a rise in serum TSH concentrations, but maintain normal levels of serum T4 (and free T4). In humans, this condition is called "subclinical" or mild hypothyroidism and generally does not require treatment.

In about half of cats, this rise in TSH is transient and will normalize within a few months as the normal part of the thyroid gland recovers from radioiodine treatment.

About  5% of the cats we treat will develop a high serum TSH concentration together with a low serum T4 level. This combination is eferred to as “overt” hypothyroidism. It is still possible that the hypothyroidism in this subset of cats will resolve, but is not as likely as with subclinical disease. We recommend close monitoring of these cat, but if overt hypothyroidism persists for longer than 3 months, then supplementation with thyroid hormone is advised.

Please note that other clinics that do not use thyroid scintigraphy for thyroid volume estimation can easily overdose the cats with the administered radioioidne, and rates of hypothyroidism can be as high as 50%, with over half of these cats having overt hypothyroidism.

Radioiodine destroys the thyroid tumor by being taken up by the thyroid gland and irradiating thyroid tissue. If there isn't enough normal thyroid tissue left (it is replaced by thyroid tumor) or the normal tissue takes up too much of the dose, hypothyroidism will occur. In human medicine, virtually all patients treated will develop hypothyroidism and are treated with thyroid hormone. With our protocol, overt hypothyroidism is rare but does occur. Unfortunately, we cannot generally predict which cats will develop hypothyroidism (unless very large doses are given on purpose, as we do in cats with large tumors or thyroid cancer).

We can't say much to predict remission of hypothyroidism based on a single testing period. We need at least two thyroid testing periods at 3-6 month intervals (measuring at least serum T4 and TSH) to predict recovery or permanent hypothyroidism.

A thyroid tumor causing hyperthyroidism is worse than hypothyroidism (no thyroid tumor) in our opinion.

Most cats do not have any signs. Some will develop lethargy or excessive hair shedding, but these are relatively nonspecific signs. The most worrisome issue associated with hypothyroidism is that it tends to make underlying kidney disease worse and it may enhance progression of concurrent kidney disease. By treating hypothyroid cats with azotemic, we can generally lower the serum kidney values and may slow long-term progression of kidney disease.

Yes, we would like to remain involved in your cat's recovery and long-term care.   If your veterinarian emails us your cats recheck laboratory results and clinical history, Dr. Peterson will review the results of all follow-up thyroid monitoring and provide your veterinarian (and you) with his interpretation of the test results and recommendations.