Radioiodine (I-131) therapy and the incidence of hypothyroidism.

Radioiodine (I-131) is one of the accepted forms of treatment for thyrotoxicosis. The major drawback of this treatment is the definite incidence of hypothyroidism, which is either early or late, and is cumulative. We set out to analyse the local incidence of this outcome. Patients with hyperthyroidism, treated with radioiodine from 1984 to end of 1989, and still on follow-up with the Department of Nuclear Medicine, Singapore General Hospital were analysed for the incidence of hypothyroidism. Three hundred and eighty-two patients were analysed (305 females, 77 males). Their ages ranged from 14 to 85 years (mean 48.2, SD 11.1 years). The doses of radioiodine ranged from 4 to 18 mCi (mean 7.6 mCi, SD 1.8 mCi). Two hundred and sixty-two patients who received I-131 only required a single dose, while 87 needed one repeated dose of I-131, and 33 needed more than 2 doses. Of the 382 patients, 271(70.9%) became clinically hypothyroid, 6 (1.6%) were subclinically hypothyroid, 100 (26.7%) were euthyroid and 3 (0.8%) remained hyperthyroid. One hundred and eighty-one patients (47.4%) were clinically hypothyroid at the end of 1 year, and an average rise of 4.8% per year was observed from the second to the fifth year. From the sixth year onwards, the average incidence of hypothyroidism was 1% per year. I-131 is indeed an effective means of controlling thyrotoxicosis (99.2% were no longer hyperthyroid). The main problem with this treatment is hypothyroidism, and this is easily managed with L-thyroxine.