Establishment of Reference Intervals of Thyrotropin and Free Thyroid Hormones during The First Week of Life

Serum concentrations of TSH, freeT4 (f-T4) and freeT3 (f-T3) were determined simultaneously in 199 fullterm newborn babies from 12 to 168 hours after birth. In addition, cord sera from 29 fullterm neonates were analyzed similarly. Cord blood concentrations of TSH were higher, and of f-T3 were lower than seen in adults and f-T4 was similar to the adult range. Serum concentrations of TSH showed a marked increase during the first day of life followed by a decline to the adult range on day four. The free thyroid hormone values increased to the maximum levels within 24 to 48 hours followed by a decrease, but remained higher than in adults even at 168 hours. The f-T3/f-T4 ratio was found to be lower than in adults except between 12 hours and 24 hours. These data allow us more detailed discussion with regard to hormonal dynamics in the newborn. Furthermore, our study is the first to successfully establish the reference intervals for TSH, f-T4 and f-T3 during the first seven days of life utilizing the most widely used kits and sufficient numbers of samples. These references will be of great assistance for prompt and proper evaluation of neonates at risk for thyroid dysfunction.