Incidence of hypothyroidism occurring long after iodine-131 therapy for hyperthyroidism.

We have studied the long-term incidence of hypothyroidism in 4,473 formerly hyperthyroid patients given I-131 therapy between 1951 and 1975. The mean age at the first I-131 treatment was 56 yr. Six percent developed hypothyroidism within one year of therapy, and 72% within 26 yr. Prior antithyroid medication did not affect the incidence of hypothyroidism. Patients cured with one dose of I-131 had a lower cumulative long-term incidence of hypothyroidism than those requiring more than one dose.