Comparison of thyroid function in each trimester of pregnancy with the use of triiodothyronine uptake, thyroxine iodine, free thyroxine, and free thyroxine index

Abstract

Evaluation of thyroid function in pregnancy was carried out by comparison of changes in the first, second, and third trimesters of pregnancy with those in nonpregnant control subjects. One hundred first-trimester patients, 84 second-trimester patients, 107 third-trimester patients, and 72 control subjects had determinations run for Murphy-Pattee T₄ iodine (T₄-1), triiodothyronine (T₃) uptake, and free thyroxine. A free thyroxine index (per cent of T₃ uptake×T₄-1) was calculated for each subject and compared to the free thyroxine values. The T₃ uptake values fell and the T₄-1 values rose progressively in each trimester. Comparison of the results of free thyroxine and the free thyroxine index showed that the index was not proportional to free thyroxine in the second and third trimesters. The free thyroxine index fell as the duration of pregnancy increased. The free thyroxine values remained remarkably stable throughout pregnancy. Free thyroxine values exhibited a small increase during the first and second trimesters but dropped back to control values by the third trimester. It is suggested that free thyroxine is the most convenient thyroid function test for use in pregnancy since the range of normal values is the same for all trimesters and for nonpregnant control subjects.