Maternal-to-fetal transfer of thyrotropin-releasing hormone in vivo.
OBJECTIVE
Our purpose was to determine the transplacental transfer of thyrotropin-releasing hormone at the time of fetal blood sampling.


METHODS
Four hundred micrograms of thyrotropin-releasing hormone was given intravenously to 13 pregnant women between 24 and 35 weeks' gestation and maternal-to-fetal transfer of thyrotropin-releasing hormone was determined at fetal blood sampling 1 to 93 minutes later. The fetal thyrotropic response to thyrotropin-releasing hormone was determined by measuring thyroid-stimulating hormone, thyroxine, and prolactin. For comparison, endogenous fetal and maternal levels of thyrotropin-releasing hormone, thyroid-stimulating hormone, thyroxine, and prolactin levels were determined in a further 20 patients undergoing fetal blood sampling between 19 and 35 weeks' gestation. The concentration of thyrotrophin-releasing hormone was measured by radioimmunoassay and thyroid-stimulating hormone, thyroxine, and prolactin by chemiluminescence assay.


RESULTS
Thyrotropin-releasing hormone was undetectable in the maternal circulation, whereas endogenous levels were detectable in the fetus from 19 weeks' gestation (median 150; range 50 to 276 pmol/L) and did not correlate with gestational age. After thyrotropin-releasing hormone injection as an intravenous bolus, peak levels in the mother were attained at 3 minutes (50,000 pmol/L). Maximal transplacental transfer of thyrotropin-releasing hormone occurred within 5 minutes of maternal administration but accounted in fetal blood for only 0.01% of initial dose administered (median 250; 30 to 550 pmol/L). Thyrotropin-releasing hormone-stimulated fetal peak thyroid-stimulating hormone levels occurred within 13 minutes and were higher than maternal values (p < 0.001). There was no change in fetal prolactin level with thyrotropin-releasing hormone therapy.


CONCLUSIONS
Although maternally administered thyrotropin-releasing hormone crosses the placenta sparingly, it still elicits a thyroid-stimulating hormone but not a prolactin response in the human fetus.
Bajoria
R
R
Royal Postgraduate Medical School, Institute of Obstetrics and Gynaecology, Queen Charlotte's and Chelsea Hospital, London, United Kingdom.
Peek
M J
MJ
Fisk
N M
NM
D000328:Adult; D003430:Cross-Sectional Studies; D005260:Female; D005312:Fetal Blood / Q000378:metabolism; D005865:Gestational Age; D006801:Humans; D007700:Kinetics; D008431:Maternal-Fetal Exchange; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D011388:Prolactin / Q000097:blood; D011446:Prospective Studies; D013973:Thyrotropin-Releasing Hormone / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D013974:Thyroxine / Q000097:blood
D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D013974:Thyroxine