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 hormone is available to the conceptus during the first trimester of pregnancy as well. Transvaginal ultrasound-guided puncture of the embryonic cavities was performed during the first trimester of pregnancy to obtain coelomic fluid between 6 and 11 weeks, and amniotic fluid between 8 and 11 weeks of pregnancy. T4 was found in coelomic fluid with mean values (+/- SEM) being 961 +/- 193 pmol T4/L (747 +/- 150 pg/mL). Concentrations increased both with gestational age and with rising maternal serum T4. Concentrations of 3,5,3'-triiodothyronine (T3) were at least 30 times lower, and those of 3,3',5'-triiodothyronine (rT3) four times higher, than coelomic fluid T4. Thyroxine and rT3 in amniotic fluid (8-11 weeks) were markedly lower than in the coelomic fluid, and T3 was undetectable. These results show that maternal thyroxine can cross the placental barrier as early as the second month of pregnancy. T4 from the coelomic fluid may reach the embryo via the yolk sac. This finding raises the possibility that the increase in maternal T4 occurring during the first trimester may be functionally important for the developing embryo, when its thyroid is not yet functioning.