Description:
<jats:title>Summary</jats:title><jats:p><jats:bold>Objective </jats:bold> To investigate the risk of hypothyroidism after radioiodine (<jats:sup>131</jats:sup>I) treatment for hyperfunctioning thyroid nodules.</jats:p><jats:p><jats:bold>Design </jats:bold> Retrospective analysis of patients treated with <jats:sup>131</jats:sup>I for hyperfunctioning thyroid nodules and followed up for a maximum of 20 years.</jats:p><jats:p><jats:bold>Patients </jats:bold> A total of 346 patients treated with <jats:sup>131</jats:sup>I in the years 1975–95, for a single hyperfunctioning nodule.</jats:p><jats:p><jats:bold>Measurements </jats:bold> Hypothyroidism was defined as TSH levels > 3·7 mU/l. Kaplan–Meier survival analysis was used to analyse permanence of euthyroidism after <jats:sup>131</jats:sup>I. A stepwise Cox proportional hazard model was used to identify factors influencing the progression to hypothyroidism.</jats:p><jats:p><jats:bold>Results </jats:bold> The cumulative incidence of hypothyroidism was 7·6% at 1 year, 28% at 5 years, 46% at 10 years and 60% at 20 years. Age (<jats:italic>P <</jats:italic> 0·01), 24‐th <jats:sup>131</jats:sup>I uptake (<jats:italic>P <</jats:italic> 0·05) and previous treatment with methimazole (MMI, <jats:italic>P</jats:italic> < 0·1) were associated with a faster progression towards hypothyroidism, while thyroid and nodule size, thyroid status at diagnosis and degree of extranodular thyroid parenchymal suppression had no influence. In hyperthyroid patients with partial parenchymal suppression, however, previous MMI treatment was the most important prognostic factor (<jats:italic>P <</jats:italic> 0·01).</jats:p><jats:p><jats:bold>Conclusions </jats:bold> After 20 years of follow‐up, 60% of patients treated with <jats:sup>131</jats:sup>I for a single hyperfunctioning nodule are hypothyroid. Factors increasing the risk of hypothyroidism are age, <jats:sup>131</jats:sup>I uptake and MMI pretreatment. The prognostic value of this last factor, however, depends on the degree of suppression of the extranodular thyroid parenchyma at the scan.</jats:p>