Thyrotropin-releasing hormone-degrading activity of neonatal human plasma

Thyroid physiology changes rapidly during the neonatal period. Thus, there is an abrupt release of TSH which peaks within 30 min after birth and there are significant increases in the plasma concentrations of T3 and T4, which peak 24--48 h after birth. In the present work, high pressure liquid chromatography was used to study the evolution of TRH-degrading activity in the plasma of newborns during the first 5 days of life. TRH-degrading activity is initially absent in cord blood and appears after 3 days of life. It reaches maternal or nonpregnant adult euthyroid plasma levels after 5 days. The exact role of this phenomenon is unknown. A possible effect of plasma T3 levels on the enzymatic degradation of TRH is postulated.