Conparative kinetics of levothyroxine and total thyroid extracts in newborns selected by the screening for the hypothyroidism

Abstract

1 - Detected as hypothyroid newborns were given either levothyroxine (1 T4) 5 mcg/kg, or bovine thyroid extract (BTE) 1ctg/kg in a single oral dose. With parental approuval, five controls subjects were choosen. TSH, T4, T3, free T4 and TBG were measured in the serum before and 1/2, 1, 1 1/2, 6, 12, 24, and 48 hours after ingestion of the drug. 2 - Before intake of the drug and compaired with controls, TSH was strongly elevated TBG was in the normal range, T3 in the lower limit of the normal and T4 and F. T4 were wery low. 3 - After levothyroxine, T4 reached to a normal value within 30 mn, increased at a maximal peak at 9th hour and decreased slowly until 24 hours remaining in the normal range. T3 concentration increased progressively and remained elevated, TSH demonstrated no changes. 4 - After BTE, serum T4 remained below the normal values for 24 hours, and Increased slowly to on optimal concentration reached only at the of the first week of daily therapy.T3 concentration was normal within 1 hour. TSH remained an change. 5 - TSH concentration continued to be very elevated during several months of therapy with BTE, but returned to normal values within two weeks with levothyroxine.These data are discussed with regards of T4/TBG binding, peripheral T4/T3 conversion and suggest that levothyroxine must be prefered as neonatal substitutive therapy.