Measurement of free thyroxine (T4) levels in pregnancy

Free T4 and TSH were measured in 119 pregnant women. Twenty of these a omen 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 f ree T4 fell as pregnancy progressed but even at the early stages of pregnan cy many measured free T4 concentrations were below the lon er end of the no npregnant normal range. This remained the case when the measurement was mad e by a dialysis kit method, These results challenge the current widely-acce pted statement that free T4 concentrations remain within the normal nonpreg nant range during pregnancy. This study highlights the need for each labora tory to develop its own normal range for free T4 levels in pregnancy.