Low-normal concentrations of free thyroxin in serum in late pregnancy: physiological fact, not technical artefact

Free thyroxin (FT4) concentrations, total thyroxin/thyroxin-binding globulin (T4/TBG) ratios, and thyrotropin (TSH) and albumin concentrations were measured in serum in a longitudinal study in each of the three trimesters of 25 normal pregnancies. In late pregnancy, FT4 estimates by assays reputedly either affected or unaffected by albumin were in the lower half of the reference range for nonpregnant subjects. T4/TBG ratios and albumin concentrations were similarly lower. FT4 overall was significantly (P less than 0.001) correlated with these latter two values. Serum TSH concentrations increased as FT4 declined in late pregnancy. Nonesterified fatty acid (NEFA) concentrations were too low to displace T4 from its binding proteins and were not correlated with other measurements. Within any one of the trimesters, FT4 and T4/TBG were independent of variations in TBG or albumin concentrations. This implies that lower FT4 concentrations in late pregnancy are real, merely coinciding with parallel decreases in albumin. They are not artefacts of albumin-affected assays.