Iodine deficiency and real-life supplementation ineffectiveness in Polish pregnant women and its impact on thyroid metabolism

Abstract

Introduction: Iodine is a pivotal component of thyroid hormones, and its deficiency leads to negative pregnancy outcomes. Therefore, during gestation, additional iodine supplementation is recommended.

Objectives: By evaluating a group of women from western Poland, the study updated on iodine status during pregnancy and the effectiveness of iodine supplementation in relation to the maternal and neonatal thyroid function.

Patients and methods: A total of 91 women were recruited before the delivery between 2019 and 2021. During the medical interview, the patients declared their dietary supplements intake. Thyroid parameters (TSH, ft3, ft4, a-TPO, a-Tg, and TRAb) were measured in the serum of mothers and in the cord blood of newborns after birth. Urinary iodine concentration (UIC) and urine/creatinine (UIC/crea) ratio were assessed in single urine samples using a validated high-performance liquid chromatography with ultraviolet detection (HPLC-UV). Neonatal TSH screening from dried blood spot was analyzed.

Results: Pregnant women showed a median (interquartile range) UIC of 106 (69-156) µg/liter and UIC/crea ratio of 104 (62-221) µg/g, whereas approximately 20% had UIC/crea below 50 µg/g, indicating iodine deficiency. The iodine supplementation ratio was 68%. No significant differences in UIC, UIC/crea and thyroid parameters were found between iodine supplemented and non-supplemented groups; however, the highest ioduria was detected when iodine was supplemented in addition to levothyroxine in comparison with both substances administered separately. Patients with UIC/crea within 150-249 µg/g demonstrated the lowest TSH and a-TPO levels. Screening TSH was above 5 mIU/liter in 6% of children.

Conclusions: Despite the national salt iodization and the recommendation to supplement iodine during gestation, the status of the abovementioned microelement and real-life intake revealed the ineffectiveness of the current iodine-deficiency prophylaxis model in pregnancy.

Keywords: hypothyroidism; iodine; micronutrients at pregnancy; neonatal cord blood; pregnancy supplementation guidelines; selenium; thyroiditis; urinary iodine concentration (UIC).

Figure 1

Comparison of ioduria in the last trimester of pregnancy in women who confirmed or denied iodine supplementation. Higher median UIC and wider confidence interval (95% CI) were observed in the iodine supplementing group, although the result was not significant (p = 0.55). Both intervals are below the normal range established by the World Health Organization (< 150 μg/liter). Medians (symbols), 95% CI (error bars). UIC, urinary iodine concentration.

Figure 2

Maternal urinary iodine concentration (A) and iodine/creatinine ratio (B) in relation to levothyroxine and/or iodine supplements intake. Pregnant women differed with respect to iodine (I) and levothyroxine (LT4) intake and were divided into three groups; I only (group 1), LT4 only (group 2), or the combined I and LT4 intake (group 3). No significant differences in urinary iodine concentration (UIC, p = 0.18, KW-H = 3.46) and iodine/creatinine ratio (UIC/crea, p = 0.09, KW-H = 5.84) between the groups were observed. However, the highest UIC or UIC/crea were observed in the group with the combined LT4 therapy and I supplementation. Medians (symbols and numbers), 95% CI (error bars) are shown. UIC/crea, urine/creatinine ratio; LT4, levothyroxine; I, iodine; KW-H, the result of Kruskal–Wallis test.