This study compares the accuracy of the free thyroxine index and free serum thyroxine radioassay in evaluating the metabolic status of 36 euthyroid women, 17 of whom were taking oral contraceptive drugs. Results indicate that the free thyroxine index does not uniformly compensate for estrogen-induced elevations in total serum thyroxine concentrations and that the free serum thyroxine is the test of choice in assessing the functional thyroid status of women taking oral contraceptive drugs.
PIP: A comparative laboratory study was conducted to assess the comparability of FTI (free thyroxine index) and measured free T4 (thyroxine) levels in 36 euthyroid women. The 36 women in the study were not pregnant; 17 of them were taking OCs (oral contraceptives). The frequency distribution of free T4 values were comparable for the OC users and the nonusers. There was a definite highly significant bimodal distribution in values for FTI between the 2 groups, with the values higher in the OC group. The lack of agreement between the 2 measurements--FTI and free T4--was probably due to the failure of the T3% RU (T3% resin uptake) to adequately compensate for estrogen-induced elevations in serum thyroxine binding proteins and total serum T4 levels. The FTI only partially compensates for estrogen-induced elevations in total T4 concentrations, resulting in abnormally high FTI values in many euthyroid women using OCs. The free T4 test was not influenced by elevated serum thyroxine binding protein levels and is, therefore, a more useful test in screening for thyroid dysfunction during pregnancy or estrogen therapy.