Low Iodine Intake May Decrease Women’s Fecundity: A Population-Based Cross-Sectional Study

Abstract

Salt iodization is one of the most cost-effective strategies to eliminate iodine deficiency disorders (IDD). However, China’s dismantling of salt monopoly has reduced the availability of iodized salt in the susceptible population in pregnancy, which might cause IDD and have adverse health effects on both themselves and their offspring. The aim of our study was therefore to explore the association between IDD and women’s reproductive health. This is a population-based cross-sectional study conducted in 2018 in Zhejiang Province, China. A total of 1653 pregnant women participated in this study. Median urinary iodine concentration (UIC) in the population was used to assess iodine intake. Cox regression analyses were used to estimate the association between iodine intake and time to pregnancy, which was indicated with fecundability ratio (FR) and 95% confidence interval (CI). The percentage of participants with iodine deficiency who had been waiting longer than 13 months to get pregnant (20%; median UIC 119.6 μg/L) was significantly higher than those with iodine sufficiency (14%; median UIC 147.1 μg/L). A significant decrease in fecundity was observed in participants with iodine deficiency (FR, 0.820; 95% CI, 0.725−0.929) than those with iodine sufficiency. These findings indicate the importance of ongoing monitoring of iodine nutrition in women of reproductive age. Keeping a safe and optimal level of iodine nutrition during pregnancy should be emphasized.

Pre-Conceptional Anti-Thyroid Antibodies and Thyroid Function in Association with Natural Conception Rates

“…However, an important research gap is that none of these studies evaluated thyroid autoimmunity or thyroid function, which may mediate the link between iodine intake and pregnancy rate. Although these previous studies [ 14 , 15 ] suggest a lower pregnancy rate in women with lower iodine intake, it is unknown whether high iodine intake is associated with a lower natural pregnancy rate.…”

“…Thus, high and low iodine intake may be associated with a decreased probability of conception through altered thyroid function and autoimmunity. A population-based cross-sectional study targeting pregnant women in China found that women with iodine deficiency (i.e., urinary iodine concentration < 150 µg/L) were more likely to report a time to pregnancy of longer than 13 months [ 14 ]. The longer time to pregnancy and lower probability of conception among women with lower iodine intake remained similar when the analysis was limited to women who conceived naturally [ 14 ].…”

“…A population-based cross-sectional study targeting pregnant women in China found that women with iodine deficiency (i.e., urinary iodine concentration < 150 µg/L) were more likely to report a time to pregnancy of longer than 13 months [ 14 ]. The longer time to pregnancy and lower probability of conception among women with lower iodine intake remained similar when the analysis was limited to women who conceived naturally [ 14 ]. In the Chinese study, the median urinary iodine concentrations were 119.6 µg/L in the coastal regions and 147.1 µg/L in the inland regions [ 14 ].…”

“…The longer time to pregnancy and lower probability of conception among women with lower iodine intake remained similar when the analysis was limited to women who conceived naturally [ 14 ]. In the Chinese study, the median urinary iodine concentrations were 119.6 µg/L in the coastal regions and 147.1 µg/L in the inland regions [ 14 ]. Similarly, in a population-based prospective cohort study targeting women in the United States trying to conceive naturally, those with lower urinary iodine concentrations (<50 µg/g creatinine) were less likely to conceive than those with higher iodine levels (≥100 µg/g creatinine) [ 15 ].…”