ESTIMATION OF FREE THYROXINE INDEX IN THE NEWBORN USING MICRO-METHODS

Abstract. Rogowski, P., Siersbæk-Nielsen, K. and Mølholm Hansen, J. (Medical Department E, the Obstetrical Department, and the Department of Clinical Chemistry, Frederiksberg Hospital, Copenhagen, Denmark). Estimation of free thyroxine index in the newborn using micro-methods. Acta Paediat Scand, 63: 201, 1974.–Thyroid function in the newborn has been studied with the purpose of establishing normal values for total plasma thyroxine, T-3 test and free thyroxine index in the neonatal period using new micro-methods. Total thyroxine determinations were carried out using the Sephadex column method (Tetralute®) which requires 25 to 50 µ1 plasma. The unbound TBG binding sites were evaluatid using a T-3 Sephadex retention test (Trilute®) requiring 50 μl plasma. Free thyroxine index were calculated as the product of the two tests. 202 fullterm newborns were examined in the period 19 to 71 hours after birth and the normal range (95 % limits) for plasma thyroxine were found to be 9.2–26.0 μg/100 ml. Normal values for the T-3 test varied between 42.5 and 64.9 % and free thyroxine index values between 510–1378 arbitrary units. The mean values of total thyroxine, T-3 test and free thyroxine index were found to be significantly increased compared with cord blood and adult mean values indicating a physiological thyroid hyperfunction in the neonatal period. The new thyroid function tests used in the present study were found to be technical simple and are suggested to be used whenever thyroid diseases in the newborn are suspected.