Radioiodine-131 treatment of thyrotoxicosis: dose required for and some factors affecting the early induction of hypothyroidism.

The results of sodium iodide 131I treatment of thyrotoxicosis in 1,168 patients (302 males, 866 females; 58.5% diffuse and 41.5% multinodular toxic goitre) are presented. At the end of the 1st year post-treatment, 54.4% were hypothyroid, and the incidence of hypothyroidism after the 2nd year increased by 3% per year. When the results were analysed according to the calculated radiation dose the thyroid, it was found that the cumulative incidence of hypothyroidism from 6 months to 2 years post-treatment rose almost proportionally to the dose in cases of doses of 1,500-15,000 rad, but increased very little for higher doses; however, the long-term incidence of hypothyroidism was almost independent of the thyroid dose. Multivariate analysis showed that the results of 131I therapy at 6 months depended also on sex (treatment being more effective in women), the consistency of the thyroid gland and the year of treatment, with the same radiation dose giving a higher incidence of hypothyroidism in patients treated recently, in comparison to those treated early in the period studied. Of the patients treated in the period 1978-1982 (mean dose, 300 microCi/g), 93.5% were cured with a single dose of 131I, and 78% were hypothyroid at 6 months post-treatment.