Lay summary
Adequate iodine intake is essential during pregnancy to produce thyroid hormones for maternal health and physical and mental development of the fetus. Iodine deficiency is a problem throughout New Zealand and the current guidelines are that women commence iodine supplementation only once pregnancy is confirmed. However, this might be too late. Paradoxically, the thyroid responds to sudden high concentrations of iodine by transiently inhibiting synthesis of thyroid hormones. We hypothesise that for iodine deficient women who start iodine supplementation during pregnancy, thyroid hormone production decreases potentially compromising optimal development of the fetus. We will recruit pregnant women and assess iodine intake and when iodine supplementation commenced and compare this with thyroid function at three times during pregnancy. We will investigate if iodine status in early pregnancy is associated with impaired thyroid function. It is essential to understand if the current guidelines could adversely affect infants.