Plasma Free Triiodothyronine Response to Thyrotropin-Releasing Hormone to Predict the Remission of Graves’ Disease Treated with Antithyroid Drugs*

Abstract

The responses of both plasma TSH and free T3 (FT3) to TRH were examined in 31 patients with Graves' disease who were euthyroid after treatment with antithyroid drugs, 6 patients with primary hypothyroidism, and 14 control subjects. TSH was measured 0, 15, 30, 60, 90, and 120 min and FT3 was measured 0, 30, 60, 90, 120, 150, and 180 min after TRH injection (500 microgram, iv). The increment in FT3 above the basal level (delta FT3) in normal controls ranged from 1.2-3.7 pmol/L, with a mean +/- SD of 2.2 +/- 0.8 pmol/L. The mean (+/- SD) delta FT3 in patients with primary hypothyroidism was 0.3 +/- 0.2 pmol/L. After the TRH test, antithyroid drugs were stopped in patients with Graves' disease. Nine of 31 Graves' patients relapsed within 6 months after the TRH test. The other 22 patients with Graves' disease were followed while in remission during the observation period of up to 48 months. The mean (+/- SD) delta FT3 were significantly lower in 9 Graves' patients who relapsed than in those who achieved remission (0.5 +/- 0.3 vs. 2.6 +/- 1.1 pmol/L; P less than 0.01). Eight of 9 Graves' patients who relapsed showed lower delta FT3 values than the lowest value (1.1 pmol/L) in 22 Graves' patients in remission. Although the mean increment of TSH above the basal level (delta TSH) was also significantly different between the Graves' patients who relapsed and those in remission (1.4 vs. 12.3 mU/L; P less than 0.01), there was considerable overlap between the 2 groups. These findings suggest that delta FT3 reflects the endocrinological recovery of the pituitary-thyroid axis and is a beneficial indicator for the termination of antithyroid drugs in Graves' disease.