Iodine is an essential nutrient for the synthesis of thyroid hormones. Insufficient iodine intake leads to hypothyroidism, which has adverse impacts on folliculogenesis, ovulation, and maturation of the corpus luteum, and ultimately influences fertility.
According to the authors of the current study, iodine's impact on women’s fertility is often overlooked. It is known that severe IDD poses a significant risk to reproduction, including spontaneous abortion, stillbirth, premature birth, and a decrease in fecundability (potential to bear children). However, there is limited knowledge concerning the damage to women’s reproductive health, particularly fertility, due to mild IDD.
The present study aimed to understand the association between mild iodine deficiency and women’s reproductive abnormalities, mainly focusing on fecundability ratio (FR), which are the odds of conception.
This is the first study to investigate the effect of iodine deficiency on the time required to conceive among women in China.
Data concerning urinary iodine concentration (UIC) of pregnant women between 2015 and 2017 were drawn from the Zhejiang Electronic Surveillance Reporting System. Iodine intake was assessed according to the criteria of the World Health Organization.
From March to December 2018, a cross-sectional retrospective study was conducted among pregnant women participating in the Zhejiang surveillance on IDD.
At community healthcare centers, 6126 married pregnant women in any trimester who had been living in the local areas for at least one year were enrolled. The women were required to provide samples of spot urine and household cooking salt to determine UIC and salt iodine concentration.
In interviews, the women were asked about assistance to conceive, time to conception, history of pregnancies, age, weight, thyroid diseases, and other potential confounders.
A total of 1653 eligible participants were included for the final analyses.
The median time to get pregnant was longer for participants with iodine deficiency (5 months) than those with iodine sufficiency (4 months), though no significant differences were observed (p = 0.266).
However, the percentage of participants with iodine deficiency who had been waiting longer than 13 months to get pregnant (20%) was significantly higher than those with iodine sufficiency (14%).
To further explore the association between iodine inadequacy and FR, the team performed Cox regression analysis. The results, before adjustment of covariables, showed that compared to participants having iodine sufficiency, iodine-deficient participants were significantly less likely to get pregnant.
After the adjustment of the potentially significant covariables (age, prepregnancy BMI, income, occupation, education, and history of spontaneous abortion), Cox proportion hazard regression analyses showed that iodine inadequacy had significantly adverse effects on FR.
Based on their model, the fecundability ratio decreased by 19.4% among iodine-deficient women aged 30.5 years, with average BMI 21.7 kg/m2, education level higher than 14 years, having domestic workers, and no history of spontaneous abortion, compared with similar women with iodine sufficiency
The researchers conclude: "Our study showed that women with mild iodine deficiency showed a positive association with risk of prolonged time to pregnancy...
"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."
They add that further studies with prospective design, diversity of population and detailed information are warranted to validate the findings and elucidate the corresponding biological mechanism.
Xing, M.; Gu, S.; Wang, X.; Mao, G.; Mo, Z.; Lou, X.; Li, X.; Huang, X.; Wang, Y.; Wang, Z.
"Low Iodine Intake May Decrease Women’s Fecundity: A Population-Based Cross-Sectional Study"