Normal maternal thyroid function during pregnancy is essential for fetal development and depends upon an adequate supply of
iodine. Little is known about how
iodine status is associated with
preterm birth and small for gestational age (SGA) in mildly
iodine insufficient populations. Our objective was to evaluate associations of early pregnancy serum
iodine,
thyroglobulin (Tg), and
thyroid-stimulating hormone (TSH) with odds of
preterm birth and SGA in a prospective, population-based, nested case-control study from all births in Finland (2012-2013). Cases of
preterm birth (n = 208) and SGA (n = 209) were randomly chosen from among all singleton births. Controls were randomly chosen from among singleton births that were not preterm (n = 242) or SGA (n = 241) infants during the same time period. Women provided blood samples
at 10-14 weeks' gestation for serum
iodide, Tg and TSH measurement. We used logistic regression to estimate odds ratios (
ORs) and 95% confidence intervals (CIs) for
preterm birth and SGA. Each log-unit increase in serum
iodide was associated with higher odds of
preterm birth (adjusted OR = 1.19, 95% CI = 1.02-1.40), but was not associated with SGA (adjusted OR = 1.01, 95% CI = 0.86-1.18). Tg was not associated with
preterm birth (OR per 1 log-unit increase = 0.87, 95% CI = 0.73-1.05), but was inversely associated with SGA (OR per log-unit increase = 0.78, 95% CI = 0.65-0.94). Neither high nor low TSH (versus normal) were associated with either outcome. These findings suggest that among Finnish women,
iodine status is not related to SGA, but higher serum
iodide may be positively associated with
preterm birth.