DETECTION OF THYROID-HORMONES IN HUMAN EMBRYONIC CAVITIES DURING THE FIRST TRIMESTER OF PREGNANCY

Transfer of maternal thyroxine (T4) to the human fetus near term, has recently been demonstrated. We investigated whether maternal thyroid h ormone is available to the conceptus during the first trimester of pre gnancy as well. Transvaginal ultrasound-guided puncture of the embryon ic cavities was performed during the first trimester of pregnancy to o btain coelomic fluid between 6 and 11 weeks, and amniotic fluid betwee n 8 and 11 weeks of pregnancy. T4 was found in coelomic fluid with mea n values (+/- SEM) being 961 +/- 193 pmol T4/L (747 +/- 150 pg/mL). Co ncentrations increased both with gestational age and with rising mater nal serum T4. Concentrations of 3,5,3'-triiodothyronine (T3) were at l east 30 times lower, and those of 3,3',5'-triiadothyronine (rT3) four times higher, than coelomic fluid T4. Thyroxine and rT3 in amniotic fl uid (8-11 weeks) were markedly lower than in the coelomic fluid, and T 3 was undectable. These results show that maternal thyroxine can cross the placental barrier as early as the second month of pregnancy. T4 f rom the coelomic fluid may reach the embryo via the yolk sac. This fin ding raises the possibility that the increase in maternal T4 occurring during the first trimester may be functionally important for the deve loping embryo, when its thyroid is not yet functioning.