Reference values for the concentration of free thyroxine and free triiodothyronine in the serum of euthyroid children

: Concentrations of free thyroxine (n = 341, male = 205, female = 136) and free triiodothyronine (n = 318, male = 198, female = 120) were determined using the radioimmunoassay method in blood serum of euthyroid children. Beyond the 30. day of life no significant differences were found in different age groups for the free thyroxine and free triiodothyronine concentration. The free thyroxine values of the infants in the range of 16.5-19.5 pg/ml decreased after the 30. day of life to the range of 15-17.5 pg/ml. In opposition to this trend free triiodothyronine values increased after the 30. day of life from the range of 2.5-3.3 pg/ml up to the range of 4.7-5.5 pg/ml.