Changes in thyroid function across adolescence: a longitudinal study.

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There are no large, longitudinal studies of thyroid function across adolescence. The aims of this study were to examine longitudinal trends in TSH, fT3 and fT4 and determine age-specific reference ranges.Thyroid function was assessed in 3415 participants in the Brisbane Longitudinal Twin Study at age 12, 14 and 16, using the Abbott ARCHITECT immunoassay. Longitudinal analyses were adjusted for body mass index and puberty.In girls, mean fT4 (± SE) increased between age 12 and 14 (by 0.30 ± 0.08 pmol/L, P <0.001), while remaining unchanged in boys; from age 14 to 16, fT4 increased in both girls (by 0.42 ± 0.07 pmol/L, P <0.001) and boys (0.64 ± 0.07 pmol/L, P <0.001). Free T3 increased slightly from 12 to 14 years in girls (by 0.07 ± 0.03 pmol/L; P = 0.042) with a more marked increase in boys (0.29 ± 0.03 pmol/L, P < 0.001) then decreased from age 14 to 16 in both sexes (girls, by 0.53 ± 0.02 pmol/L, P <0.001; boys, by 0.62 ± 0.03 pmol/L, P <0.001). From age 12 to 14, TSH showed no significant change in girls or boys, then increased from age 14 to 16 in both sexes (girls, by 4.9%, 95% CI 2.4 to 10.3%, P = 0.020; boys, by 7.2%, 95% CI 3.0 to 11.6%, P=0.001). Reference ranges differed substantially from adults, particularly for fT4 and fT3.Thyroid function tests in adolescents display complex, sexually dimorphic patterns. Implementation of adolescence-specific reference ranges may be appropriate.

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