Prevalence of Thyroid Dysfunction in Postpartum Women With Suboptimal Iodine and Selenium and Adequate Iron Status

Ying Jin; Jane Coad; Shao J. Zhou; Sheila Skeaff; Thiagarajah Ramilan; Louise Brough

Disclosures

Clin Endocrinol. 2021;95(6):873-881. 

In This Article

Results

A total of 87 breastfeeding mother-infant pairs were recruited at three months postpartum, nine participating pairs dropped out from the study at six months postpartum (n = 78). A description of characteristics of breastfeeding women and their infants at enrolment are shown in Table 2. At six months postpartum, 31% (24/78) were exclusively breastfeeding, 4% (3/78) had stopped breastfeeding, with the remainder providing partial breastfeeding. Iodine, selenium and iron-containing dietary supplements were used by nine, one and six women, respectively.

Thyroid Volume and Thyroid Hormones

The median (p25, p75) maternal total thyroid volume was 6.1 (4.4, 8.4) ml, ranging from 2.2 to 15.2 ml, with none above 18 ml which is the suggested cut-off of thyroid enlargement for women.[12] Based on maternal thyroid function markers, 18% (13/74) had thyroid dysfunction [3% (2/74) had overt hypothyroidism and 15% (11/74) had subclinical hyperthyroidism], with 8% (6/74) having positive TPOAb indicating autoimmune thyroid disorders (Table 3). Out of six women with positive TPOAb, one was classified as overt hypothyroidism, three were classified as subclinical hyperthyroidism, and two were classified as euthyroid. Positive TPOAb was correlated with abnormal TSH (r = 0.259, p =.029). Total thyroid volume was weakly, positively associated with serum-free T4 (r = 0.317, p =.004).

Iodine, Selenium and Iron Status

The World Health Organization (2007) defines iodine deficiency in a population by using the median UIC and suggests a cut-off for deficiency in non-pregnant women below 100 μg/L.[20] Maternal median (p25, p75) UIC was 85 (43, 134) μg/L, indicating deficiency. Median BMIC was 59 (39, 109) μg/L, below 75 μg/L which has been suggested as the index of sufficient infant iodine intake;[21] for exclusively breastfeeding women, a suggested reference range for BMIC is 60 to 465 μg/L with a median of 171 μg/L.[22] After excluding nine women with positive Tg antibodies, median Tg was 11.4 (8.6, 18.6) μg/L, above the suggested cut-off of 10 μg/L, and suggests iodine deficiency.[13] All three biomarkers indicated iodine deficiency in this cohort of postpartum women. Median maternal plasma selenium concentration was 105.8 (95.6, 115.3) μg/L; with 23% (17/74) below 95 μg/L, suggesting inadequate status as this concentration is required to saturate GPX activity.[14]

During childbirth, 17% (13/78) of women experienced severe blood loss (> 500 ml), of whom 46% (6/13) received an iron transfusion and 15% (2/13) had a blood transfusion. Six women reported taking iron-containing supplements of either 60 mg (5/6) or 5 mg (1/6) daily. Mean Hb was 132.5 ± 9.00 g/L. Median (p25, p75) SF was 41 (27, 78) μg/L after excluding three women with elevated CRP. Median sTfR was 1.12 (1.00, 1.26) mg/L, with 4% having elevated values suggesting iron-deficient erythropoiesis.[23] In total, 90% (64/71) of women were classified as having sufficient iron stores. Four participants were classified as having anaemia without iron deficiency. Only three women were classified as being ID, and no participants were classified as having IDA.

Women who had a plasma selenium concentration below 95 μg/L had a significantly lower TSH, higher free T4 and a higher serum Tg, compared with women having an adequate plasma selenium concentration (Table 4).

Iodine, Selenium and Iron Status and Thyroid Hormone Concentration

All nine women using iodine-containing supplements (ranging from 100 to 250 μg iodine per day) had normal TSH concentrations, while 21% (13/62) who did not use iodine-containing supplements had abnormal TSH concentrations. The proportion of women with thyroid dysfunction was higher in those who had plasma selenium concentrations below 95 μg/L compared with women who had plasma selenium concentrations of 95 μg/L or above (46.7% vs. 10.7%, p =.004).

The logistic regression showed a negative but significant association between plasma selenium concentration and abnormal TSH concentration, and the marginal effect suggests women with lower plasma selenium concentration were 1.12% times more likely to have abnormal TSH concentrations. However, none of other covariates had a significant effect when entered the model (Table 5).

No significant correlations were observed between biomarkers for iron status (Hb, SF and sTfR) and thyroid hormone concentrations. There were no statistically significant differences in iron status observed between women with abnormal and normal TSH concentrations.

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