Preterm birth and risk of medically treated hypothyroidism in young adulthood

Objective Previous studies suggest that low birth weight is associated with thyroid autoimmunity and hypothyroidism in later life, but the potential effect of preterm birth, independent of foetal growth, is unknown. Our objective was to determine whether preterm birth is independently associated with medically treated hypothyroidism in young adulthood. Design/Participants National cohort study of 629 806 individuals born in Sweden from 1973 through 1979, including 27 935 born preterm (<37 weeks). Measurements Thyroid hormone prescription during 2005-2009 (ages 25.5-37.0 years), obtained from all outpatient and inpatient pharmacies throughout Sweden. Results Preterm birth was associated with increased relative odds of thyroid hormone prescription in young adulthood, after adjusting for foetal growth and other potential confounders. This association appeared stronger among twins than singletons (P = 0.04 for the interaction). Twins had increased relative odds across the full range of preterm gestational ages, whereas singletons had increased relative odds only if born very preterm (23-31 weeks). Among twins and singletons, respectively, adjusted odds ratios for individuals born preterm (<37 weeks) were 1.54 (95% CI, 1.11-2.14) and 1.08 (95% CI, 0.98-1.19), and for individuals born very preterm (23-31 weeks) were 2.62 (95% CI, 1.30-5.27) and 1.59 (95% CI, 1.18-2.14), relative to full-term births. Conclusions This national cohort study suggests that preterm birth is associated with an increased risk of medically treated hypothyroidism in young adulthood. This association was independent of foetal growth and appeared stronger among twins than singletons. Additional studies are needed to confirm these new findings in other populations and to elucidate the mechanisms.