Longitudinal study of serum thyroid hormone levels during normal pregnancy

We undertook a prospective longitudinal study of thyroid function in 60 normal pregnant women and measured serum concentrations of T 4, triiodothyronine (T 3), T-uptake, thyroxine binding globulin (TBG), free thyroxine index (FTI), free T 4, albumin and thyrotropin (TSH). From these data we established reference ranges for each of these analytes for each trimester and examined the interrelationships between laboratory measurements of thyroid function tests. We observed significant increases in serum concentrations of thyrotropin and decreases in free T 4, assays commonly used as first line investigations of thyroid activity during pregnancy. However, the 95th centile intervals for both analytes remained within the reference range for nonpregnant women.