Association between age-specific preconception thyroid-stimulating hormone (TSH) and birth weight: a retrospective study.

OBJECTIVE: Maternal thyroid function affects fetal birth weight and age is an important factor in regulating thyroid function. Thus, we aimed to explore the association between age-specific preconception thyroid-stimulating hormone (TSH) and birth weight. DESIGN: Cohort study. PARTICIPANTS: A total of 97 755 preconception Chinese women aged 20-39 years old from the National Free Preconception Checkups Project were included. SETTING: Participants were divided into four age groups: 20-25 years, 25-29 years, 30-34 years and 35-39 years. The preconception TSH levels within 6 months before pregnancy and fetal birth weight, including large for gestational age (LGA), small for gestational age (SGA) and appropriate for gestational age (AGA), were collected and analysed using restricted cubic spline regression. Logistic regression investigated the relationship between various TSH groups and birth weight. RESULTS: (1) Preconception TSH levels differed among four age groups; (2) in the 20-24 years group, preconception TSH was associated with the incidence of LGA and AGA (p<0.001). The TSH range for OR of LGA lower than 1.0 was 1.32-2.12 mIU/L and OR of AGA higher than 1.0 was 1.80-2.06 mIU/L; (3) there were no significant associations between age-specific TSH and risks of LGA, SGA or AGA in the 25-29 years, 30-34 years and 35-39 years groups; (4) compared with TSH 1.32-2.12 mIU/L, lower (<1.32 mIU/L) and higher (>2.12 mIU/L) TSH were associated with a higher risk of LGA and lower incidence of AGA in 20-24 years. CONCLUSION: Preconception TSH exhibited a significant association with LGA and AGA in the 20-24 years age group, but not in the 25-39 years age group. Young preconception women should not neglect paying attention to their thyroid function, associated with the risk of LGA.