Current normal values for TSH and FT3 in children are too low: evidence from over 11,000 samples

Background: Current pediatric normal values for thyroid function tests are based on data from relatively few patients. Objective: To develop new normal pediatric values based on a large sample of children. Patients and methods: Data were collected from a computerized database in Jerusalem, Israel, of thyroid function tests taken in community pediatric clinics. Samples from patients with antithyroid peroxidase antibodies and/or antithyroglobulin antibodies and from those treated with levothyroxine, methimazole, propylthiouracil, thyrotropin, lithium, phenobarbital, or glucocorticoids were excluded. The analysis included over 11,000 samples tested for TSH, free triiodothyronine (FT3), and free thyroxine (FT4) with the ADVIA (R) Centaur (TM) system. Results: The upper normal limit (UNL) for TSH increased by about 1 mIU/L and the lower normal limit (LNL) for FT3 increased by 0.5-2 pmol/L in different age groups. There was no significant change in FT4 values. Conclusions: These reference data should replace current normal values.