Transient hypothyroidism after iodine-131 treatment for thyrotoxicosis.

Transient hypothyroidism after iodine-131 treatment for thyrotoxicosisMany patients develop permanent hypothyroidism after being treated with iodine-131 (131I) for thyrotoxicosis and require replacement therapy with thyroxine.'Transient hypothyroidism has been observed after subtotal thyroidectomy for thyrotoxicosis,2 but whether this phenomenon exists after 1311 treatment is disputed.3In 1977 we reported two cases of transient hypothyroidism after 131I treatment,4 since when we have seen a further eight cases, which we report here. Patients, methods, and resultsThe patients comprised seven women, median age 52 years (range 49-69), and three men aged 50, 59, and 67 years attending the thyroid clinic at this hospital.Thyrotoxicosis was confirmed biochemically and by an increased two-hour uptake of 1321.None of the patients were receiving antithyroid drugs, and all had diffuse thyroid enlargement.Doses of 131I ranged from 2 5 to 11 8 mCi (median 4 mCi).The diagnosis of transient hypothyroidism was based on a low serum thyroxine (T4) concentration.Serum T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) con- centrations were measured by radioimmunoassay.Normal ranges were: serum T4 60-140 nmol 1 (4 7-10 9 yg 100 ml), serum T3 1 2-2 7 nmol l (0 8-1 8 ng ml), and serum TSH < 6 mU 1. Statistical analysis included Friedman's two-way analysis of variance test and Wilcoxon's matched pairs signed ranks test.The figure shows the serum concentrations of T4, T3, and TSH measured immediately before 1311 treatment, when subnormal serum T4 concentrations were first found, and when serum T4 concentrations were first noted to be normal again.No patient became clinically hypothyroid when the serum T4 concentration was transiently low.After treatment the median serum T4 concentration decreased from 211.5 nmoll1 (16 4 tg/ 100 ml) (range 160-320 nmol 1; 12 4-24 9 fig / 100 ml) to 38 5 nmol/l (3 0 tsg/l00 ml) (range 20-55 nmol'l; 1 6-4 2 ,ug, 100 ml) and