There is little information for very low birth weight infants on levels of thyroid hormones, pituitary-thyroid responsiveness, and effects of in utero exposure to TRH. We assayed total T3 and TSH in plasma of infants 24-32 wk gestation and examined responsiveness to a TRH stimulation test at 28 days. Mothers in preterm labor at 9 centers received glucocorticoid plus either TRH(400 μg q 8h) or placebo and blood samples were collected from infants at birth, 2h, and on days 1, 3, 7, 14, 21 and 28. In the placebo group (n=137), T3 and TSH at birth were 62.1±3.3 (se) ng/dl and 5.9±0.3 mU/L, respectively. At 2 h of age, T3 and TSH increased 2- and 3-fold, respectively; at 3-7 days levels decreased to ≈85 ng/dl and 2 mU/L and at 28 days were 125 ng/dl and 3.7 mU/L. Infants of 24-28 wk had lower levels of T3 than 28-32 wk infants at all time points but were similarly responsive to TRH at 28 days. In the TRH group (n=131), infants exposed to TRH ≤6 h before birth had elevated T3 (1.3-fold vs control) and TSH (3.3-fold) at birth, whereas those treated 7-36 h before delivery had lower T3 (78% of control) and TSH (54%) during the first 24 h but levels were not different from control at 3-28 days. Responsiveness to a TRH stimulation test was similar in 82 control and 85 TRH-treated infants at 28 days: basal TSH was 3.7±0.4 vs 4.0±0.5, TSH 30 min after TRH infusion was 17.4±1.5 vs 20.6±1.6, number of hyper-responders (30 min TSH >35 mU/L) was 5 and 8, and number of non-responders (30 min ≤TSH basal) was 3 and 1, respectively. We conclude that extremely premature infants have less rise in plasma TSH and T3 immediately after birth and maintain lower T3 concentrations during the first month of life. The stimulatory and suppressive effects of antenatal TRH treatment observed at birth are transient and do not affect pituitary-thyroid responsiveness at 28 days.