Development of thyrotropin circadian rhythm in infancy.

Normal children and adults show a similar pattern of diurnal variation of TSH secretion with lower values at 1100 h and higher around 2300 h. The purpose of this study was to investigate the age of appearance of TSH circadian rhythm. In 57 fullterm infants 0-6 months old and in 37 premature infants 1-4 weeks old TSH was measured at 1030, 1100, 1130 h and 2230, 2300, and 2330 h. No diurnal rhythm was detected in both premature and fullterm infants less than 4 weeks of life. After the first month of life a significant difference between AM and PM values was observed in fullterm infants. In infants 1-2 months old mean +/- SEM AM and PM values were 2.8 +/- 0.2 and 3.5 +/- 0.4 mU/L, respectively (P less than 0.025), in infants 3-4 months old 3.0 +/- 0.6 and 4.1 +/- 0.8 (P less than 0.01) and in infants 5-6 months old 1.8 +/- 0.2 and 2.6 +/- 0.3 (P less than 0.0005). These data clearly indicate that the development of TSH circadian rhythm starts after the first month of life.