Thyroid function in the long-term follow-up of patients treated with iodine-131 for thyrotoxicosis.

In February, 1972, 58% of patients euthyroid after iodine-131 therapy for thyrotoxicosis between 1954 and 1966 had a raised plasma thyroid-stimulating-hormone (T.S.H.) (greater than 7-4 mU/l) and 42% a normal T.S.H. level. A group of 69 of the euthyroid patients with a raised plasma T.S.H. (25-0 +/- 2-0 mU/l) in 1972 was re-examined annually for three years. There was no apparent change in the mean plasma T.S.H. level between 1972 and 1975 in the patients remaining euthyroid, but overt hypothyroidism developed in 3 patients in 1973, in a further 3 patients in 1974, and in 1 patient in 1975. In contrast, none of a group of 61 patients, euthyroid with a normal plasma T.S.H. (4-0 +/- 0-2 mU/l) in 1972, developed overt hypothyroidism over the next three years, although slightly raised T.S.H. levels were recorded in 3 patients in 1974 and in a further 6 patients in 1975. Both the mean serum T-4 and T-3 in the euthyroid patients with a raised plasma T.S.H. were significantly lower, but still in the respective normal ranges, than those in the euthyroid patients with a normal plasma T.S.H. No significant difference in the fasting serum-cholesterol or triglyceride levels could be demonstrated between the two groups. Since no patient with a normal plasma T.S.H. after iodine-131 treatment for thyrotoxicosis six to eighteen years earlier developed overt hypothyroidism over a three-year period, the follow-up of such patients need not be so frequent as that of similarly treated euthyroid patients with a raised plasma T.S.H. in whom overt hypothyroidism develops at the rate of 2-5% per year.