Figure 2.Serum thyroid hormone binding globulin (TBG), percent saturation of TBG, Free T4, and Free T3 levels from 606 normal pregnancies in Brussels, showing the progressive rise in serum TBG during the first part of gestation , accompanied by a a progressive decrease in the free T4 index (saturation level of TBG by T4), and free T4 and T3 concentrations. (From Glioner D. The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocrine Revs 1997;18:404.)