Plasma TSH and serum T-4 levels in long-term follow-up of patients treated with 131-I for thyrotoxicosis.

In February 1972 58% of patients euthyroid after iodine-131 therapy given for thyrotoxicosis between 1954 and 1966 had a high plasma TSH (>7.4 muU/ml) and 42% a normal plasma TSH level. A group of 69 of the euthyroid patients with high plasma TSH levels (25.0+/-2.0 muU/ml) in 1972 were re-examined 15 and 24 months later. The mean plasma TSH in the 66 patients remaining euthyroid at 15 months was 22.6+/-1.8 muU/ml, while three patients had become hypothyroid. At 24 months 64 of the patients were still available for study, of whom 61 remained euthyroid with a mean plasma TSH of 21.6+/-2.0 muU/ml, and a further three had become hypothyroid.All of a group of 61 of the euthyroid patients with normal plasma TSH levels (4.0+/-0.2 muU/ml) in 1972 remained euthyroid at 24 months with a mean plasma TSH of 4.1+/-0.3 muU/ml, though the plasma TSH level had become slightly raised in three.The mean serum T-4 level in the euthyroid patients with a high plasma TSH was significantly lower, though still in the normal range, than that in the euthyroid patients with a normal plasma TSH both in 1972 and in 1974.Since no patient with a normal plasma TSH level after iodine-131 treatment six to 18 years earlier for thyrotoxicosis developed hypothyroidism over a two-year period, the follow-up of such patients need not be so rigorous as that of similarly treated euthyroid patients with raised plasma TSH levels in whom hypothyroidism developed at the rate of 5% per year.