Hypothyroidism After Low-Dose ¹³¹I Treatment of Hyperthyroidism

Sixty-four patients with hyperthyroidism were treated with a single dose of 3 mCi or less of 131I before 1951. Nine were lost to follow-up shortly after treatment. The other 55 were found, their thyroid status at last examination noted, and the incidence of hypothyroidism calculated, using the life-table method. Few were hypothyroid (3.7%) at the end of the first year, and the incidence remained low (7.5%) after 5 years. Thereafter, however, the incidence increased annually at a rate of 3.4% yearly from the fifth to the seventeenth year, reaching a total of 48% at the end of this time. The rate of increase was similar to that noted by others, using larger doses of radioiodine. Although the number of patients studied was small, the data suggest that low doses of radioiodine, compared with conventional doses, result in a lower incidence of hypothyroidism at 1 year and a delay in the appearance of hypothyroidism but no change in the later annual increment in the incidence of hypothyroidism and perhaps no effect on the ultimate incidence of hypothyroidism.