Plasma Thyroid Hormones in Premature Infants: Effect of Gestational Age and Antenatal Thyrotropin-Releasing Hormone Treatment

Thyroid hormones are important for both perinatal adaptation and long-term psychomotor development; however, there is limited information on the effects of extreme prematurity and antenatal TSH-releasing hormone (TRH) treatment on pituitary-thyroid function. In this study we assayed plasma triiodothyronine (T3) and TSH in infants who were part of a collaborative trial of antenatal maternal TRH therapy. Within the control population (n = 166), infants of 24-28-wk and 28-32-wk gestational age had comparable levels of T3 (0.94 and 1.06 nmol/L, respectively) and TSH (5.7 and 7.2 mU/L) at birth, but the increases at 2 h and subsequent T3 levels were less in the 24-28 wk versus 28-32-wk gestation infants. In the TRH-treated group (n = 131), T3 was lower in the first day for infants delivered 7-72 h after antenatal TRH compared with control infants. TSH at birth was approximately 3.5-fold greater for infants delivered at 0-6 h after the last TRH dose compared with the control group and was suppressed in infants delivering at 7-36 h. T3 and TSH levels were not different between control and TRH-treated groups at 3-28 d of age. In TRH stimulation tests on d 28, control and TRH-treated groups had similar peak levels of TSH and incidence of exaggerated response (TSH > or = 35 mU/L). We conclude that extremely premature infants have a reduced postnatal surge in TSH and T3 and maintain lower T3 concentrations, probably reflecting tertiary hypothyroidism. The stimulatory and suppressive effects of antenatal TRH treatment observed at birth are transient and do not affect pituitary-thyroid responsiveness at 28 d of age.