Iodine-131 treatment of hyperthyroidism: significance of effective half-life measurements

Our goals were to evaluate the effect of half-life determination and differences in the half-life of 131I between patients with Graves' disease and toxic nodular goiter, and the influence of antithyroid drugs on iodine uptake.

We reviewed the records of 555 patients who had received radioiodine treatment for Graves' disease and toxic nodular goiter to analyze iodine uptake, half-life values and pretreatment with antithyroid drugs. Two different methods of dose calculation were compared: one using repeated uptake measurements at 24 and 48 hr and 4 or 6 days to define the effective half-life. The other method assumed a half-life of 5 days and uptake at 24 hr only. All patients were treated according to the first method. A follow-up questionnaire was sent to 327 patients (238 responders) to assess the treatment outcome.

After comparing the results of the two methods, we found that repeat uptake measurements and determination of effective half-life results in administered activities that differ considerably from those calculated when an assumed, fixed half-life and a single uptake measurement are used. The simpler method would lead to over- as well as undertreatment of the patient. There was a functional difference between patients with Graves' disease and toxic nodular goiter, as reflected by the shorter 131I half-life in Graves' disease (mean 5.0 days) than toxic nodular goiter (mean 6.0 days) and a skewed distribution in toxic nodular goiter. Patients pretreated with antithyroid drugs had shorter 131I half-lives in both categories. Ten percent of the patients required more than one treatment; 94% of the patients with Graves' disease and 45% with toxic nodular goiter had thyroxine substitution 1-5 yr after treatment.

A dose calculation method that uses three uptake measurements provides sufficient data about the effective half-life of 131I in the thyroid. There is considerable difference in the half-life based on the disease being treated (Graves' disease or toxic nodular goiter). The 131I half-life also is shorter after pretreatment with anti-thyroid drugs. Thus, the simpler method leads to significant uncertainty, leading to over- as well undertreatment of the patient.