Establishment of reference intervals for fT3, fT4, and TSH levels in Japanese children and adolescents

The present study aimed to establish new reference intervals (RIs) for serum free triiodothyronine (fT3), free thyroxine (fT4), and thyroid stimulating hormone (TSH) levels in Japanese children and adolescents aged 4 to 19 years. A total of 2,036 (1,611 girls, 425 boys) participants were included over a 17-year period; they all tested negative for antithyroid antibodies (TgAb, TPOAb) and were found to have no abnormalities on ultrasonography. RIs were determined by nonparametric methods. The results showed that serum fT3 was significantly higher in the 4-15-year-olds than in the 19-yearolds. The serum fT4 was significantly higher in the 4-10-year-olds than in the 19-year-olds. The serum TSH was significantly higher in the 4-12-year-olds than in the 19-year-olds. All of them gradually decreased with age to approximate the adult levels. The upper limit of TSH was lower in those aged 13 to 19 years than in adults. The differences were examined by sex. The serum fT3 was significantly higher in boys than in girls between the ages of 11 and 19 years. The serum fT4 was significantly higher in boys than in girls between the ages of 16 and 19 years. There did not seem to be any sex difference in those under 10 years of age. In conclusion, serum fT3, fT4, and TSH levels in children and adolescents differ from those in adults. It is important to evaluate thyroid function using the new RIs that are appropriate for chronological age.