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Context: Serum thyroid hormone levels differ between children and adults, but have not been studied longitudinally in the same individuals through childhood.
Objective: To assess changes in thyroid stimulating hormone (TSH) and thyroid hormone levels over childhood and their inter-relationships.
Design: Cohort study.
Setting: The Avon Longitudinal Study of Parents and Children a population-based birth cohort.
Participants: 4,442 children who had thyroid function measured at age 7, and 1,263 children who had thyroid function measured at age 15. 884 children had measurements at both ages.
Main Outcome Measures: Reference ranges for TSH, free tri-iodothyronine, free thyroxine, their longitudinal stability and the relationships between them.
Results: Children at age 7 years had a higher FT3 (6.17 pmol/l, 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 FT3 above the adult reference-range. Higher FT3 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 FT3 at age 7 and age 15 even after adjusting for confounders. In contrast TSH was negatively associated with FT4 at both ages.
Conclusions: There are substantial changes in TSH and thyroid hormone levels over childhood and their relationships, in particular for FT3 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.
- thyroid hormones
- reference values
- thyroid function
- pituitary gland
- free triiodothyronine assay
- measures of outcome