Iodine Nutrition and Iodine Supplement Initiation in Association with Thyroid Function in Mildly-to-Moderately Iodine-Deficient Pregnant and Postpartum Women

Abstract

Background Whereas the adverse effects of severe iodine deficiency during pregnancy are well documented, the effects of mild-to-moderate deficiency are not well established. Objectives We aimed to explore whether iodine nutrition and timing of iodine supplement initiation are associated with thyroid function in pregnant and postpartum women. Methods In this cohort study, 137 pregnant women were enrolled and followed up at gestational weeks (GWs) 18 and 36, and 3 and 6 mo postpartum. Thyroid function tests [thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), and free thyroxine (fT4)], urinary iodine and creatinine concentration (UIC:Cr), and iodine intake (including iodine supplement use) were measured at each time point. The associations between thyroid hormone concentrations and UIC:Cr, iodine intakes, and iodine supplement use were estimated using multiple generalized estimating equation models. Results The median UIC at GW18 was 94 μg/L, indicating mild-to-moderate iodine deficiency. UIC:Cr (β; 95% CI) per 100 μg/g was negatively associated with fT3 (−0.191; −0.331, −0.051) and fT4 (−0.756; −1.372, −0.141) concentrations. Iodine intake (β; 95% CI) per 100 μg/d was positively associated with TSH (0.099; 0.022, 0.177), and negatively associated with fT3 (−0.084; −0.0141, −0.027) and fT4 (−0.390; −0.599, −0.182) concentrations. Compared with no use of supplement, those initiating an iodine-containing supplement prepregnancy and continuing through pregnancy had lower TSH (estimated means) (1.35 compared with 1.68 mIU/L, P = 0.021), and higher fT3 (4.48 compared with 4.28 pmol/L, P = 0.035) and fT4 (15.2 compared with 14.4 pmol/L, P = 0.024) concentrations. Conclusions Lower iodine availability during pregnancy and postpartum was associated with lower TSH, and higher fT3 and fT4 concentrations. The use of an iodine-containing supplement that was initiated prepregnancy and continuing through pregnancy was associated with lower TSH, and higher fT3 and fT4 concentrations, which may suggest improved thyroid function. These findings support the notion that optimization of iodine intake should start before pregnancy. This trial was registered at clinicaltrials.gov as NCT02610959.

Life-course perspectives of milk consumption among young Norwegian women and their knowledge of milk as a source of iodine: a qualitative study

“…The majority of the plant-based milk alternatives that are on the rise in the Norwegian market are not iodine enriched ( 6 ). In Norway, mild to moderate iodine deficiency has been found among pregnant and lactating women ( 1 , 7 ) together with limited knowledge of iodine and health ( 8 – 10 ). Yet, little attention has been given to investigating young women’s attitudes toward milk, their milk consumption practices, and their knowledge about the potential health consequences of omitting milk from the diet ( 1 – 3 , 7 ).…”

An update: maternal iodine supplementation, thyroid function tests, and child neurodevelopmental outcomes

“…A 2021 observational study in China comparing women who took iodine supplements in preconception and pregnancy to those who did not found a positive effect on maternal T4 [52 ▪▪ ]. An observational study in Norway in a MMID area found MIS started in the preconception period had a similar beneficial effect with lower TSH and higher T3, T4 [53 ▪ ]. An RCT of 200 pregnant patients with MMID in Sweden found MIS to have a positive effect on maternal thyroglobulin and UIC, but not TSH or T4 [54 ▪▪ ].…”

A Scoping Review of Iodine and Fluoride in Pregnancy in Relation to Maternal Thyroid Function and Offspring Neurodevelopment

“…Three observational studies of I intake during pregnancy were completed in Norway. The first study in Norway assessed dietary I intake, UIC, and thyroid function in a cohort of pregnant women ( n = 1730) [ 79 ] during the 2nd and 3rd trimesters with I insufficiency by both UIC and dietary intake (94 μg/L and 85 μg/L and 202 μg/d and 153 μg/d, respectively) [ 79 ]. Among women taking an I-containing supplement before pregnancy and throughout pregnancy, TSH level was lower and T3 and T4 levels were higher than those not taking a supplement [ 79 ].…”

“…The first study in Norway assessed dietary I intake, UIC, and thyroid function in a cohort of pregnant women ( n = 1730) [ 79 ] during the 2nd and 3rd trimesters with I insufficiency by both UIC and dietary intake (94 μg/L and 85 μg/L and 202 μg/d and 153 μg/d, respectively) [ 79 ]. Among women taking an I-containing supplement before pregnancy and throughout pregnancy, TSH level was lower and T3 and T4 levels were higher than those not taking a supplement [ 79 ]. The study suggests that I intake can positively influence thyroid function in populations of pregnant women with mild-to-moderate I deficiency.…”