Conception is followed by a series of hormonal and metabolic changes that involve most maternal endocrine systems. With regard to thyroid metabolism, these include an increase in serum thyroxine-binding globulin and thyroid hormone concentrations, increased renal clearance of iodine, and increased production and turnover of thyroxine (T)¹. Fetal and maternal thyroid physiology differ, but the systems interact by means of the placenta and amniotic fluid, which modulate the transfer of iodine and small but important amounts of thyroid hormone from mother to fetus¹,². In this article we shall review recent data and new insights regarding the changes . . .