Serum free thyroxine and free triiodothyronine concentrations in pregnancy.

Blood was taken from women in each trimester of pregnancy at routine antenatal visits. Further samples were collected from an age matched control group of euthyroid women taking no medication. Free triiodothyronine and free thyroxine concentrations were measured by direct radioimmunoassay (Amerlex Kit methods, Amersham International, Amersham, Buckinghamshire).2 These free hormone assays use thyroxine and triiodothyronine derivatives labelled with iodine-125 that have been chemically modified to inhibit severely their binding to serum thyroid hormone binding proteins but that bind normally to thyroxine or triiodothyronine antibodies. In the assays free thyroid hormones and the modified labelled derivatives compete for binding to high affinity thyroxine or triiodothyronine antibodies, and the proportion of labelled derivative bound to the antibody is inversely related to the serum free thyroxine or free triiodothyronine concentration. Total serum triiodothyronine and thyroxine concentrations were measured by in house radioimmunoassay, thyroxine binding globulin concentration by immunoelectrophoresis, and thyroxine binding prealbumin and albumin concentrations by immunodiffusion. Predicted free triiodothyronine and free thyroxine concentrations were derived from total hormone and binding protein concentrations, using a method taking into account the known binding characteristics of the thyroid hormone binding proteins.3 Serum thyroxine binding globulin and total thyroxine concentrations rose during pregnancy, whereas concentrations of free thyroxine and free triiodothyronine were progressively lower in each trimester (table). The changes in measured free thyroxine and free triiodothyronine were paralleled by a reduction in predicted free hormone concentrations.