Response of the maternal, fetal, and neonatal pituitary-thyroid axis to thyrotropin-releasing hormone

Thyrotropin releasing hormone (TRH) readily crosses the placenta and stimulates the fetal pituitary. We studied the response of the maternal and fetal pituitary-thyroid axes to TRH and the influence of prenatal exposure to TRH on the physiological postnatal increase in thyrotropin (TSH) and triiodothyronine (T3) in the neonate. Twenty-six pregnant women received TRH (400 or 600 ng) intravenous or saline (controls) either 2 or 12 h before elective cesarean section at term. Administration of 400 jug of TRH resulted in significant elevations of maternal TSH (15.7 ± 2.9 versus 3.2 ± 0.4 μU/ml, р < 0.01) and prolactin (416 ± 94 versus 223 ± 41 ng/ml, p < 0.05) 2 h later. Maternal T₃ remained unchanged. A higher dose of TRH (600 μg) produced comparable results. Maternal administration of TRH (400 jug) 2 h before delivery resulted in significant increases in fetal TSH and T₃ over controls (21.1 ± 3.7 versus 4.8 ± 1.0 μU/ml, and 132 ± 12 versus 64 ± 9 ng/dl, p < 0.01, respectively). Cord blood hormone levels 12 hours after TRH administration were similar to controls. Higher doses of TRH did not produce further increases in fetal TSH or T₃. Control and treated neonates demonstrated similar physiological postnatal increases in TSH and T₃, suggesting that prior exposure to TRH did not blunt this response. These data suggest that maternal administration of TRH is an effective way of increasing fetal T₃ levels, and that this treatment does not inhibit the postnatal surge in TSH and T₃.