Reference intervals for free thyroxine, total triiodothyronine, thyrotropin and thyroglobulin for Quebec newborns, children and teenagers

Objective: Paediatric reference values, although essential for interpreting patientsʹ results, are scarce. Moreover, they are often population- and instrument-dependent. We have measured free thyroxine (Free T4), total triiodothyronine (Total T3), thyroglobulin (Tg) and thyrotropin (TSH) in samples obtained from groups of newborns, children and adolescents. Subjects and methods: Blood samples collected from healthy children and teenagers (100 girls and 100 boys) of age groups ranging between 9–10, 11–14 and 15–17 years and selected randomly from a cohort representative of the Quebec population, were used. Samples from infants of age ranging between 1 day and 2 years (n = 99) were obtained from a hospital-based population with benign conditions unlikely to affect thyroid function. Variables were measured on the Access 2 immunosystem. Results: Free T4, Tg and TSH levels declined significantly with age. However, Total T3 level presented a nonlinear variation with age, being lower in the first month of life.