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Iodine-131 Treatment of Hyperthyroidism: Significance of Effective Half-life Measurements

Thyroid Unit, Department of Oncology, Oncology Center and Department of Medical Physics and Medical Engineering, Sahlgrenska University Hospital, Göteborg, Sweden

Correspondence: For correspondence or reprints contact: Dr. Gertrud Berg, Department of Oncology, Sahlgrenska University Hospital, S-413 45 Göteborg, Sweden.

ABSTRACT

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. Methods: 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 patents were treated according to the first method. A follow-up questionnaire was sent to 327 patients (238 responders) to assess the treatment outcome. Results: 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. Conclusion: 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.

Key Words: hyperthyroidism • iodine-131 • effective half-life • Graves' disease • toxic nodular goiter

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