During the first 20 weeks of pregnancy, serum TBG levels will increase by two fold. This causes more T3/T4 to be picked up by TBG → The body responds by producing more T3/T4.
NOTE: Though the total T3/T4 may increase, the total physiologically active T3/T4 is unchanged.
Ain KB, Mori Y, Refetoff S. Reduced clearance rate of thyroxine-binding globulin (TBG) with increased sialylation: a mechanism for estrogen-induced elevation of serum TBG concentration. J Clin Endocrinol Metab. 1987;65(4):689.
Glinoer D. The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocr Rev. 1997;18(3):404.