Reference ranges for serum thyroid hormones free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulating hormone (TSH) in children were set using the assay kits currently used in clinical settings. A total of 342 children (111 males and 231 females) who were negative for antithyroid antibodies (TgAb, TPOAb) and were found to have no abnormalities on ultrasonographic examination of the thyroid gland were divided into 6 age groups: 4-6 years (45 children), 7-8 years (40), 9-10 years (53), 11-12 years (65), 13-14 years (83), and 15 years (56) for the study. FT3, FT4 and TSH levels were determined by electrochemiluminescence immunoassay (ECLIA) (ECLusys FT3, FT4 and TSH).The reference range for FT3 (pg/mL) was 2.91-4.70 for the age group of 4-6 years, 3.10-5.10 for the age group of 7-8 years, 3.10-4.87 for the age group of 9-10 years, 2.78-4.90 for the age group of 11-12 years, 2.77-4.59 for the age group of 13-14 years, and 2.50-4.64 for the age group of 15 years . The reference range for FT4 (ng/dL) was 1.12-1.67, 1.07-1.61, 0.96-1.60, 1.02-1.52, 0.96-1.52, 0.95-1.53. The reference range for TSH (μU/mL) was 0.62-4.90, 0.53-5.16, 0.67-4.52, 0.62-3.36, 0.54-2.78, 0.32-3.00. Serum FT3, FT4 and TSH levels in children differ from those in adults. It is, therefore, of importance to perform evaluation of thyroid function in children using reference values appropriate for the chronological ages, because misdiagnosis of hypothyroidism or inappropriate secretion of TSH (SITSH) and oversight of mild subclinical hypothyroidism could occur if the diagnosis is made using reference values for adults.