Thyroid homeostasis in mother–child pairs in relation to maternal iodine status: the MISA study

BACKGROUND/OBJECTIVES: Iodine deficiency during pregnancy may influence maternal and foetal thyroid function with the risk of causing neurocognitive and psychomotor deficits in the offspring. The objective of this study was to assess iodine status in pregnant women from Northern Norway and to investigate the influence of iodine status on maternal and infant thyroid function. SUBJECTS/METHODS: Women from the Northern Norway Mother-and-Child contaminant Cohort Study (MISA) donated a blood and urine sample at three visits during their pregnancy and postpartum period (in second trimester, 3 days and 6 weeks after delivery. N=197). Women were assigned to iodine status groups according to urine iodine concentrations (UICs) in second trimester and mixed effects linear models were used to investigate potential associations between iodine status and repeated measurements of thyroid-stimulating hormone (TSH), thyroid hormones (THs), TH-binding proteins and thyroid peroxidase antibodies. Associations between maternal iodine status and TSH in heel prick samples from the infants were investigated with linear regression. RESULTS: Median UIC in second trimester was 84 μg/l (range 18–522) and 80% had UIC below recommended level (<150 μg/l). Iodine-deficient women had higher concentrations of T3, FT3 and FT4 (estimated differences (confidence intervals) of 0.10 nmol/l (0.01, 0.17), 0.16 pmol/l (0.05, 0.26) and 0.45 pmol/l (0.10, 0.78), respectively) compared with iodine-sufficient women. The concentrations varied within normal reference ranges, but the majority of women with subclinical hypothyroidism were iodine deficient. Maternal iodine status did not influence infant TSH concentrations. CONCLUSIONS: This study indicate iodine deficiency among pregnant women in Norway. Iodine status during pregnancy influences maternal thyroid homeostasis and is therefore a risk factor for foetal and infant development.