Serum soluble interleukin-2 receptors as an index of the biological activity of thyroid hormones in hyperthyroidism.

In order to examine whether serum soluble Interleukin-2 Receptors (sIL-2R) could be used as a marker of the biological effects of the thyroid hormones, we measured the sIL-2R, sex hormone binding globulin and beta-2 microglobulin levels in thirty-three hyperthyroid patients (14 with Graves' disease, 17 with Toxic Nodular Goiter and 2 with toxic adenoma) before and during treatment with antithyroid drugs. We found that serum sIL-2R concentrations of the patients, at diagnosis, were significantly higher compared with normal controls (2424 +/- 1447 vs 459 +/- 184 U/ml). All hyperthyroid patients had sIL-2R levels > mean + 2SD of normal controls, with 28 of the 33 patients having sIL-2R concentrations higher than 1011 U/ml (mean + 3SD of normal controls). Only 15 patients had SHBG levels higher than 3SD above the mean for the normal controls and 28 had SHBG levels 2SD above the mean for the normal controls. Three of the 5 hyperthyroid patients with normal SHBG levels at presentation had abnormally high sIL-2R levels. In all patients sIL-2R levels decreased gradually during therapy down to normal levels when euthyroidism was achieved. A strong positive correlation was found between sIL-2R, SHBG and T3 and T4 concentrations. Serum B2-microglobulin (B2-m) levels were higher than the upper normal limit only in 9 patients, but a significant decrement was observed in all patients when euthyroidism was achieved. The above results indicate that serum sIL-2R levels could be a useful marker of the in vivo biological effects of the thyroid hormones on lymphocytes in hyperthyroid patients.