SERUM THYROTROPIN RESPONSE TO THYROTROPIN-RELEASING-HORMONE AND FREE THYROID-HORMONE INDEXES IN PATIENTS WITH FAMILIAL THYROXINE-BINDING GLOBULIN DEFICIENCY
The response in serum thyrotropin (TSH) to synthetic thyrotropin-releasing hormone (TRH) and serum free thyroxine index (FT4I) and free triiodothyronine index (FT3I) was investigated in 6 patients with familial thyroxine-binding-globulin (TBG) deficiency. The total serum thyroxine (T4) and triiodothyronine (T3) concentrations were significantly decreased, compared with those of normal subjects (3.4 .+-. 0,9 .mu.g/dl, [mean .+-. SD] vs. 9.0 .+-. 1.5 .mu.g/dl, P < 0.01; and 87 .+-. 27 ng/dl vs. 153 .+-. 37 ng/dl, P < 0.01, respectively). FT4I was lower than the normal range in all but 1 (5.3 .+-. 1.5 vs. 8.9 .+-. 1.6, P < 0.01), whereas FT3I was all in the normal range and of no significant difference from the normal control (132 .+-. 22 vs. 148 .+-. 25). Serum TSH concentrations in TBG deficiency were all in the normal range (1.0-4.2 .mu.U[units]/ml), and the maximum TSH increments following TRH 500 .mu.g i.v. were 8.9 .+-. 2.0 .mu.U/ml and not significantly different from the normal control (10.2 .+-. 4.5 .mu.U/ml). The euthyroid state in familial TBG deficiency is probably more clearly defined by TRH-test, and the normal response to TRH in familial TBG deficiency is presumably under the control of the serum free T3 level rather than the serum free T4 level.