Maturation in Serum Thyroid Function Parameters Over Childhood and Puberty: Results of a Longitudinal Study

CONTEXT: Serum thyroid hormone levels differ between children and adults, but have not been studied longitudinally through childhood. OBJECTIVE: To assess changes in thyroid-stimulating hormone (TSH) and thyroid hormone levels over childhood and their interrelationships. DESIGN: Cohort study. SETTING: The Avon Longitudinal Study of Parents and Children, a population-based birth cohort. PARTICIPANTS: A total of 4442 children who had thyroid function measured at age 7, and 1263 children who had thyroid function measured at age 15. Eight hundred eighty-four children had measurements at both ages. MAIN OUTCOME MEASURES: Reference ranges for TSH, free tri-iodothyronine (FT(3)), free thyroxine (FT(4)), their longitudinal stability, and interrelationships. RESULTS: Children at age 7 years had a higher FT(3) [6.17 pmol/L, standard deviation (SD) 0.62] than children at age 15 (5.83 pmol/L, SD 0.74); P < 0.0001 with 23.2% of children at age 7 having FT(3) above the adult reference range. Higher FT(3) levels at age 7 in boys (P = 0.0001) and girls (P = 0.04) were associated with attainment of a more advanced pubertal stage at age 13. TSH was positively associated with FT(3) at age 7 and age 15 even after adjusting for confounders. In contrast, TSH was negatively associated with FT(4). CONCLUSIONS: There are substantial changes in TSH and thyroid hormone levels over childhood, in particular for FT(3), which appear to relate to pubertal readiness. Our data provide increased insight into the evolution of the pituitary–thyroid axis over childhood and may have implications for determining optimal ranges for thyroid hormone replacement in children.