Trimester-Specific Thyroid Function in Pregnant Women with Different Iodine Statuses

Iodine nutritional status and its associations with thyroid function of pregnant women and neonatal TSH

Introduction

Iodine serves as a crucial precursor for the synthesis of thyroid hormones and plays an import role in both pregnant women and their offspring. The relationships between iodine nutritional status and maternal thyroid function and neonatal outcomes remain inconclusive in areas with adequate iodine nutrition. This study aims to investigate their correlations.

Methods

Blood, morning urine and 24-hour urine were collected from the pregnant women to measure thyroid functions, serum iodine concentration (SIC), morning urine iodine concentration (UIC) and 24-hour urine iodine excretion (24-hour UIE). Indicators of their offspring's neonatal indexes were recorded.

Results

A total of 559 pregnant women were enrolled in this study. The iodine indicators including Tg, 24-hour UIE and morning UIC were significantly different among the euthyroid pregnant women and those with different thyroid disorders. The levels of FT3, FT4, and SIC exhibited a gradual decline and the concentration of TSH exhibited a gradual increase trend throughout the progression of pregnancy in euthyroid pregnant women. There were no significant differences in neonatal outcomes and neonatal TSH values among euthyroid pregnant women and thyroid disorders pregnant women. SIC had a significant impact on maternal FT4 levels throughout all three trimesters, with varying degrees of importance observed in each trimester. TSH level emerged as the primary determinant of FT4 during the first trimester, while SIC exerted a predominant influence on FT4 levels in the second and third trimesters. The prevalence of thyroid disorders in pregnant women was the lowest when the SIC of pregnant women was probable in the range of 60~70 μg/L, 24-hours UIE was in the range of 250~450 μg, and Tg was in the range of 9~21 μg/L. Maternal TSH exhibited a notable influence on neonatal TSH levels, particularly at the 50th and 75th quantiles. Among the iodine nutritional indicators, SIC and morning UIC demonstrated higher AUC values for abnormal FT4 and TSH, respectively.

Discussion

The iodine nutrition status of pregnant women exerts an impact on their thyroid function and prevalence of thyroid disorders, and neonatal TSH was affected by maternal TSH. SIC may be a better indicator for iodine nutritional assessment than other indexes.

Iodine status in pregnant women in northern Spain. Effect of iodised salt and iodine supplements on thyroid function

INTRODUCTION

Iodine deficiency is linked to thyroid dysfunction, particularly in pregnant women. The objective of this study was to ascertain the iodine levels of women in the second trimester of pregnancy, analysing the influence of iodine ingestion on urinary iodine concentration (UIC) and maternal thyroid function.

METHODS

A prospective observational study of pregnant women from Health Area IV of Asturias (northern Spain) recruited before 13 weeks of gestation between May and June 2017. A questionnaire on iodine intake was completed at the first visit, and urine and serum samples were collected at baseline and again during the second trimester. UIC, thyroid stimulating hormone (TSH) and free thyroxine (FT4) obtained in the second trimester of gestation were analysed and related to iodine intake. Thyroid autoimmunity was also analysed in half of the pregnant women at baseline.

RESULTS

A total of 241 pregnant women were studied. Of these, 56.7% used iodised salt, 46.7% consumed ≥2 servings of dairy products daily and 88.1% took iodine supplements. Median UIC was 191μg/l (135.3-294μg/l), with 68.1% of the women having UIC ≥150μg/l. Only iodised salt consumption provided protection against iodine deficiency (odds ratio 0.35 [0.20-0.63], p=0.001). In women with no autoimmune thyroid disease (n=88), mean levels of TSH were lower in those that consumed iodised salt than in those that did not (respectively, 2.08±0.89mIU/l vs. 2.56±1.02mIU/l, p=0.025). In women with autoimmune thyroid disease (n=30), mean levels of TSH were higher in those that took iodine supplements than in those that did not (respectively, 2.97±1.25mIU/l vs. 1.16±0.41mIU/l, p=0.002).

CONCLUSIONS

The pregnant women studied from Health Area IV in Asturias maintain adequate nutritional iodine status in the second trimester of gestation. In our sample, only the consumption of iodised salt was associated with adequate iodine nutrition, without affecting maternal thyroid function. Most of the women used iodine supplements, which was linked to higher levels of TSH in pregnant women with autoimmune thyroid disease.

Iodine Nutritional Status of Pregnant Women After 14 Years of Lipiodol Supplementation: a Cross-Sectional Study in Historically Iodine-Deficient Areas of China

Southern Xinjiang has a history of serious iodine deficiency. Since 2007, pregnant women in this area have taken iodized salt and oral lipiodol preparations to prevent iodine deficiency disorders. However, the current status of iodine nutrition and thyroid function in this population is unknown. A cross-sectional study was conducted on 555 pregnant women from 5 counties (cities) in southern Xinjiang and 429 pregnant women from 3 counties in northern Xinjiang. The participants were given a questionnaire and serum concentrations of free triiodothyronine (FT3), free thyroxine (FT4), thyrotropic stimulating hormone (TSH), thyroglobulin antibody (Tg-Ab), and thyroid peroxidase antibody (TPO-Ab), and the urine iodine concentration (UIC) was measured. The median UIC and interquartile range [M (IQR)] of the 984 pregnant women in the study was 189.38 (143.15, 288.77) μg/L. Positive Tg-Ab and TPO-Ab titers were detected in 6.74% and 9.55%, 8.30% and 9.84%, and 7.39% and 10.55% in T1, T2, and T3, respectively. The incidence of subclinical hypothyroidism, clinical hypothyroidism, and isolated hypothyroxinemia in pregnant women in areas where lipiodol pills were taken was 4.32%, 0%, and 1.44%, respectively, which was significantly lower than those in areas where lipiodol pills were not taken. The median UIC (IQR) of pregnant women in the two regions was 213.80 (130.44, 331.77) μg/L and 168.30 (155.0, 254.8) μg/L, the UIC of pregnant women who took lipiodol pills was higher than in those who did not take lipiodol pills, and the difference was statistically significant (p < 0.05). According to WHO standards, all pregnant women in southern Xinjiang are in a state of adequate iodine nutrition. Taking lipiodol pills has improved the iodine nutrition level of pregnant women in this area. The results of this study did not find that oral lipiodol pills had adverse effects on pregnant women's thyroid function, but it is necessary to further study the effect of oral lipiodol pills on the offspring.

Evaluation of iodine nutritional status during pregnancy by estimated 24-h urinary iodine excretion: population variation range and individual accuracy

Objective:

To explore the accuracy of estimated 24-h urinary iodine excretion (24-h UIE_est) in assessing iodine nutritional status.

Design:

Fasting venous blood, 24-h and spot urine samples were collected during the day. The urinary iodine concentration (UIC) and urinary creatinine concentration (UCrC) were measured, and the urinary iodine-to-creatinine ratio (UI/Cr), 24-h UIE_est, and 24-h urinary iodine excretion (24-h UIE) were calculated. At the population level, correlation and consistency between UIC, UI/Cr, 24-h UIE_est and 24-h UIE were assessed using correlation analysis and Bland–Altman plots. At the individual level, receiver operating characteristic (ROC) curves were used to analyse the accuracy of the above indicators for evaluating insufficient and excessive iodine intake. The reference interval of 24-h UIE_est was established based on percentile values.

Setting:

Indicator can accurately evaluate individual iodine nutrition during pregnancy remains controversial.

Participants:

Pregnant women (n 788).

Results:

Using 24-h UIE as standard, the correlation coefficients of 24-h UIE_est from different periods of the day ranged from 0·409 to 0·531, and the relative average differences ranged from 4·4 % to 10·9 %. For diagnosis of insufficient iodine intake, the area under the ROC curve of 24-h UIE_est was 0·754, sensitivity and specificity were 79·6 % and 65·4 %, respectively. For diagnosis of excessive iodine intake, the area of 24-h UIE_est was 0·771, sensitivity and specificity were 66·7 % and 82·0 %, respectively. The reference interval of 24-h UIE_est was 58·43–597·65 μg.

Conclusions:

Twenty-four-hour UIE_est can better indicate iodine nutritional status at a relatively large sample size in a given population of pregnant women. It can be used for early screening at the individual level to obtain more lead time for pregnant women.