Endocrine Abstracts

Background: Iodine deficiency and excess are well-recognized risk factors for thyroid dysfunction. This systematic reveiw and meta-analysis was designed, for the first time, to explore whether maternal iodine status or supplementation is associated with postpartum thyroid function.

Methods: Electronic databases, including the MEDLINE/PubMed, Web of Science, Embase, and Scopus were searched between January 1923 and December 2021 to identify relevant studies. We assessed the quality of studies using the Newcastle-Ottawa scale. The pooled mean thyroid stimulating hormone (TSH), free thyroxin (fT4), and thyroxin (T4) concentrations and 95% confidence intervals (CIs) were estimated based on maternal iodine status. Iodine status was defined based on median values <100 and ≥100 µg/l for urinary iodine concentration (UIC) or breast milk iodine concentration (BMIC) during postpartum. We used a fixed/random effect model based on the absence/presence of heterogeneity. A narrative synthesis of the data was performed for iodine supplementation.

Results: Of the 2175 studies were identified, 18 were eligible for inclusion in the meta-analysis. Thyroid hormones in women who had UIC ≥ 100 µg/l were higher than those of women with UIC <100 µg/l during the postpartum period. The pooled values [95% CI] for TSH, fT4, and T4 concentrations in iodine-sufficient women were 1.31 [1.09, 1.53] mIU/l, 14.26 [13.86, 14.66] pmol/l, and 91.97 [88.61, 95.33] nmol/l, respectively; whereas the corresponding values in iodine-deficient women were 1.00 [0.84, 1.16] mIU/l, 12.26 [10.49, 14.03] pmol/l, and 79.80 [59.53, 100.07] nmol/l, respectively. However, none of these differences was significant. The concentration of thyroid hormones in women with BMIC <100 µg/l and ≥100 µg/l were within the normal range. Iodine supplementation was administered from 1 week to 9 months postpartum as supplements doses from 75 to 300 µg oral iodine daily (n=3) or a single 400 mg dose of iodized oil (n=1). In none of these, significant differences in thyroid parameters of postpartum women between the iodine-supplemented and control groups were observed.

Conclusion: Findings of the present systematic review and meta-analysis showed no effects of iodine status or supplementation on thyroid hormones in postpartum women. Further investigations are still needed to explore the effects of different degrees of iodine deficiency as well as iodine excess on different maternal thyroid parameters during postpartum and effects on thyroid function in breastfed offspring.