Iodine Metabolism in Urine and Breast Milk among Lactating Women with Adequate Iodine.

BACKGROUND: In lactating women, iodine metabolism is regulated and maintained by the kidneys and mammary glands. Limited research exists on how iodine absorbed by lactating women is distributed between the kidneys and breasts. OBJECTIVES: This study aimed to accurately evaluate the total iodine intake (TII), urinary iodine excretion (UIE), and breast milk iodine excretion (BMIE) in lactating women and explore the relationship between TII and total iodine excretion (TIE). METHODS: A 7-d iodine metabolism study was conducted on 41 lactating women with a mean age of 30 y in Yuncheng and Gaoqing, China, from December 2021 to August 2023. TII and TIE were calculated by measuring the iodine content in food, water, 24-h urine, feces, and breast milk. The urinary iodine excretion rate (UIER), breast milk iodine excretion rate (BMIER), and partitioning of iodine excretion between urine and breast milk were determined. RESULTS: Iodine metabolism studies were performed for 285 d. The median TII and TIE values were 255 and 263 µg/d, respectively. With an increase in TII, UIER, and BMIER, the UIE and BMIE to TII ratio exhibited a downward trend. The median UIER, BMIER, and proportion of iodine excreted in urine and breast milk were 51.5%, 38.5%, 52%, and 37%, respectively. When the TII was <120 µg/d, the BMIER decreased with the increase of the TII (ß: -0.90; 95% confidence interval: -1.08, -0.72). CONCLUSIONS: When maternal iodine intake is low, the proportion in breast milk increases, ensuring sufficient iodine nutrition for infants. In addition, the UIE of lactating women with adequate iodine concentrations is higher than their BMIE. This study was registered at clinicaltrials.gov as NCT04492657.

An Iodine Balance Study in Chinese School-age Children.

CONTEXT: Few iodine balance studies have been conducted in school-age children. OBJECTIVE: This study aimed to conduct an iodine balance study in school-age children. METHODS: We measured daily iodine intake, excretion, and retention for 3 consecutive days without any dietary interventions in school-age children. Linear mixed-effects models were used to fit the relationship between total iodine intake and iodine retention. RESULTS: 29 children aged 7-12 years (mean age 10.2 ± 1.4 years) with normal thyroid function and thyroid volume were recruited. The 0 balance value (iodine intake = iodine excretion, iodine retention = 0 µg/day) shifted with iodine intake in an iodine sufficient population. The 0 balance value for school-age children with an iodine intake of 235 (133, 401) µg/day is 164 µg/day. Children aged 7-12 years with iodine intake >400 µg/day were almost all in a positive iodine state. CONCLUSION: An iodine intake of 235 (133, 401) µg/day for children aged 7-10 years achieved a 0 balance value of 164 µg/day. Long-term iodine intake of >400 µg/day is not recommended.

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

OBJECTIVES: To explore trimester-specific thyroid function changes under different iodine statuses throughout pregnancy. METHODS: A cross-sectional study was conducted to assess the pregnancy iodine status, and 2,378 healthy pregnant women covering all 3 trimesters were recruited. Urinary iodine concentration (UIC) was measured by collecting spot urine samples. Blood samples were collected to evaluate thyroid function. Thyroid B-ultrasonography was conducted to measure the thyroid volume (Tvol). RESULTS: The median UIC was 168 µg/L (111-263 µg/L). The UIC, free triiodothyronine (FT3), and free thyroxine (FT4) were significantly decreased as the pregnancy progressed (p < 0.001, p for trend <0.001), while Tvol increased (p < 0.001, p for trend <0.001). Thyrotropin (TSH) was significantly different between the 3 trimesters and showed an upward trend (p < 0.001), but the p for trend was not significant (p for trend = 0.88). After stratification by UIC, there were no significant differences in serum TSH, FT4, or FT3 level between UIC groups. Tvol was significantly higher in the UIC ≥500 µg/L group in the first trimester (ß: 2.41, 95% CI: 1.09-3.72, p <0.001), as well as in the 250 ≤ UIC < 500 µg/L group (ß: 1.65, 95% CI: 0.61-2.70, p < 0.001) and UIC ≥500 µg/L group (ß: 3.35, 95% CI: 1.96-4.74, p < 0.001) in the third trimester. CONCLUSIONS: No difference was observed in TSH, FT3, or FT4 among the different iodine status groups throughout pregnancy. Tvol increased as the pregnancy progressed, and it was especially higher in the UIC ≥500 µg/L group in the first and third trimesters.

Thyroglobulin can be a functional biomarker of iodine deficiency, thyroid nodules, and goiter in Chinese pregnant women.

BACKGROUND AND OBJECTIVES: Thyroglobulin (Tg) is considered a sensitive indicator of iodine status for children and adults, but its usefulness for pregnant women is unknown. The aim of this study was attempting to explore the relationship between Tg and iodine status and the association between elevated Tg and thyroid diseases. METHODS AND STUDY DESIGN: A total of 2163 pregnant women were recruited in this study. The ratio of urine iodine concentration and urine creatinine concentration (UI/Cr) was measured in spot urine samples. Serum thyroid hormones and thyroglobulin were measured. Thyroid nodules and thyroid volume were diagnosed by ultrasound. RESULTS: The geometric mean of serum Tg was significantly higher in the UI/Cr <100 µg/g group (10.94 [2.47] µg/L) and the UI/Cr >500 µg/g group (11.48 [2.35] µg/L) than in the 150-249 µg/g group (9.64 [2.32] µg/L). The generalized linear model analysis showed that Serum log(10) Tg concentration was much higher in the UI/Cr <100 µg/g group (ß=0.052, p=0.026) than in the 150-249 µg/g group. Multivariate logistic regression models demonstrated that elevated Tg may be a risk factor for both goiter (OR=8.30) and thyroid nodules (OR=2.73). CONCLUSIONS: Pregnant women with UI/Cr <100 µg/g have a higher Tg, and those with elevated Tg concentrations have a higher risk of thyroid nodules and goiter. Tg can be a functional biomarker of iodine deficient, thyroid nodules and goiter.

Excessive iodine intake is associated with formation of thyroid nodules in pregnant Chinese women.

In recent decades, the prevalence of thyroid nodules (TNs) has steadily increased in pregnant women. Therefore, this study aimed to investigate associations between TNs and iodine intake in pregnant women. From March 2016 to May 2017, serum and spot urine samples, as well as demographic data and medical history, were collected from 2353 pregnant women. Urine iodine concentration (UIC) and creatinine (Cr) level were determined in spot urine samples; serum thyroid hormones and thyroid autoantibodies were also determined. We examined the hypothesis that excess iodine intake influences incidence of TNs. Herein, TNs were diagnosed by ultrasonography, whereas demographic data and medical history were collected by questionnaire. Multivariate logistic regression models were used to calculate the odds ratios and their corresponding confidence intervals for TN risk factors. Generalized linear mixed model was used to assess the random effects of the regions. The UIC and UIC to creatinine ratio (I/Cr ratio) were significantly higher in pregnant women with TNs (168 and 190 µg/L, respectively, for UIC [P < .01]; 139 and 155 µg/g, respectively, for I/Cr ratio [P < .01]). Thyroglobulin, age, pre-pregnancy body mass index, and iodine-excessive region were associated with TNs (odds ratio = 1.01, 1.06, 1.04, and 2.38, respectively). Whereas I/Cr ratio was not a significant risk factor for TNs in pregnant women when adjusted for potential confounders, iodine excess (I/Cr ratio >500 µg/g) was a risk factor in pregnant women in their second trimester. People living in areas with excessive iodine in drinking water should decrease their iodine intake, and a safe water source should be provided to ensure that the I/Cr ratio of pregnant women can be maintained at an optimal level.