Plasma selenium concentration can be a significant predictor of postpartum thyroid dysfunction in women with iodine deficiency but adequate iron status, according to research results published in Clinical Endocrinology.

Because iron levels are “rarely clinically determined” in postpartum women in New Zealand  who did not experience significant blood loss during childbirth, researchers sought to investigate thyroid function in women 6 months postpartum relative to maternal iodine, selenium, and iron status.

The Mother and Infant Nutrition Investigation was an observational, longitudinal cohort study that included participants from the North Island of New Zealand. Participants included girls and women 16 years and older who had given birth to a healthy-term singleton infant less than 3 months of age who they were breastfeeding.


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The cohort included 87 mother-infant pairs, but 9 of these pairs dropped out at 6 months postpartum. At 6 months postpartum, 31% of women were breastfeeding exclusively, 4% had stopped breastfeeding, and the remaining women were providing partial breastfeeding. Dietary supplements containing iodine, selenium, and iron were taken by 9, 1, and 6 women, respectively.

The median maternal thyroid volume was 6.1 mL (range, 2.2-15.2 mL). Based on maternal thyroid function markers, 18% of women had thyroid dysfunction (3% overt hypothyroidism, 15% subclinical hypothyroidism) and 8% had positive thyroid peroxidase antibody (TPOAb) indicating autoimmune thyroid disorders. One of the 6 women with positive TPOAb had overt hypothyroidism, 3 had subclinical hyperthyroidism, and 2 were euthyroid. Positive TPOAb was correlated with abnormal thyroid-stimulating hormone (TSH), and total thyroid volume was weakly but positively associated with serum free thyroxine (T4).

Maternal median urinary iodine concentration (UIC) was 85 µg/L (range, 43-134), indicative of a deficiency. The median breast milk iodine concentration (BMIC) was 59 µg/L (range, 39-109). After the exclusion of 9 women with positive thyroglobulin (Tg) antibodies, median Tg level was 11.4 µg/L (range, 8.6-18.6), which is above the suggested cutoff of 10 µg/L for iodine deficiency.

All 3 biomarkers indicated iodine deficiency in these postpartum women.

The median maternal plasma selenium concentration was 105.8 µg/L (range, 95.6-115.3); 23% of women had levels less than 95 µg/L suggesting inadequate status.

During childbirth, 17% of women experienced severe blood loss of more than 500 mL; 46% of these women received an iron transfusion, and 15% had a blood transfusion. A total of 6 women reported consumption of iron-containing supplements of either 60 mg or 5 mg daily. Mean hemoglobin was 132.5±9.00 g/L, median serum ferritin (SF) was 41 µg/L (range, 27-78), and median soluble transferrin receptor (sTfR) was 1.12 mg/L (range, 1.00-1.26).

The researchers classified 4 participants as having anemia without iron deficiency, 3 women as being iron deficient, and no participants as having iron-deficiency anemia.

Women with plasma selenium concentrations less than 95 µg/L had significantly lower TSH, higher T4, and higher serum Tg compared with women who had adequate selenium concentrations.

All 9 women who used iodine-containing supplements (100-250 µg/d) had normal concentrations of TSH, and abnormal TSH concentrations were documented in the 13 women who did not use supplements. The proportion of women with thyroid dysfunction was higher in women who had plasma selenium concentrations less than 95 µg/L.

Results of a logistic regression analysis found a negative but significant association between plasma selenium concentration and abnormal TSH concentration; this marginal effect suggests that women with lower plasma selenium were more likely to have abnormal concentrations of TSH.

No significant correlations were noted between biomarkers or iron status and thyroid hormone concentrations, and no significant differences in iron status were seen between women with abnormal and normal TSH.

Study limitations include the small sample size, lack of generalizability to the wider population, and insufficient statistical power to evaluate the effects of selenium-containing supplements on thyroid function.

“To the best of our knowledge, the MINI study was the first to examine iodine, selenium, and iron concurrently in relation to thyroid function of postpartum women,” the researchers concluded. “A high prevalence of thyroid dysfunction…was observed, and low plasma selenium concentration significantly increased the risk of thyroid dysfunction within this group of iodine deficient postpartum women who predominantly had adequate iron status.”

“Strategies are required to improve both iodine and selenium status, which may support optimal thyroid function for women during [the] perinatal period,” they added.

Reference

Jin Y, Coad J, Zhou SJ, Skeaff S, Ramilan T, Brough L. Prevalence of thyroid dysfunction in postpartum women with suboptimal iodine and selenium and adequate iron status. Clin Endocrin (Oxf). Published online May 18, 2021. doi:10.1111/cen.14502