Measurement of free thyroxine (T4) levels in pregnancy.

Free T4 and TSH were measured in 119 pregnant women. Twenty of these women had the free T4 measured a second time during the pregnancy; 35 of the 119 had the free T4 measured by 2 different methods. The results showed that free T4 fell as pregnancy progressed but even at the early stages of pregnancy many measured free T4 concentrations were below the lower end of the nonpregnant normal range. This remained the case when the measurement was made by a dialysis kit method. These results challenge the current widely-accepted statement that free T4 concentrations remain within the normal nonpregnant range during pregnancy. This study highlights the need for each laboratory to develop its own normal range for free T4 levels in pregnancy.