[Free thyroxine values during pregnancy].

Pregnancy induces a series of physiological modifications that include a significant elevation of thyroxine-binding globulin (TBG) levels. This elevation interferes in the serum levels of total thyroxine and stresses the use of free thyroxine (FT4), together with TSH measurement, as the first line tests in the evaluation of thyroid function in pregnancy. A careful definition of the normal values for FT4 during pregnancy is paramount for the study of thyroid abnormalities, since minimal dysfunctions can result in significant fetal abnormalities.