Background: Iodine supplementation is recommended to pregnant women in iodine deficient populations, but the impact in moderate iodine deficiency is uncertain. We assessed the effect of an iodine containing prenatal multiple micronutrient supplement in a rural Gambian population at risk of moderate iodine deficiency.

Methods: This study uses data and samples collected as a part of the randomized controlled trial ENID, (Early Nutrition and Immune Development, ISRCTN49285450) conducted in Keneba, The Gambia. Pregnant women (<20 weeks gestation) were randomized to either a daily supplement of multiple micronutrients (MMN) containing 300 µg of iodine, or an iron-folic acid (FeFol) supplement. Randomisation was masked to participants and investigators. The co-primary outcomes were maternal urinary iodine concentration (UIC) and serum thyroglobulin (Tg), assessed at baseline and 30 weeks' gestation. Secondary outcomes were maternal serum thyroid stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4) (assessed at baseline and 30 weeks' gestation), breast milk iodine concentration (BMIC) (assessed at 8, 12 and 24 weeks postpartum), infant serum Tg (assessed at birth (cord), 12 and 24 weeks postpartum) and serum TSH (assessed at birth (cord)). The effect of supplementation was evaluated using mixed effects models.

Results: A total of 875 pregnant women were enrolled between April 2010 and February 2015. In this secondary analysis we included women from the MMN (n=219) and FeFol (n=219) arm of the ENID trial. At baseline, median (IQR) maternal UIC and Tg was 51 µg/L (33-82) and 22 µg/L (12-39), respectively, indicating moderate iodine deficiency. Maternal MMN supplement increased maternal UIC (p<0·001), decreased maternal Tg (p<0·001) and cord blood Tg (p<0.001) compared with FeFol. Maternal thyroid function tests (TSH, TT3, TT4, and TT3/TT4 ratio) and BMIC did not differ according to maternal supplement group over the course of the study.

Conclusion: Supplementing moderately iodine deficient women during pregnancy improved maternal iodine status and reduced Tg concentration. However, the effects were not attained postpartum and maternal and infant iodine nutrition remained inadequate during the first six months after birth. Consideration should be given to ensuring adequate maternal status through pregnancy and lactation in populations with moderate to severe iodine deficiency.