Serum iodine concentration in pregnant women and its association with urinary iodine concentration and thyroid function

Abstract

Summary Objective This study aims to evaluate the association of serum iodine concentration (SIC) with urinary iodine concentration (UIC) and thyroid function in pregnant women, as well as to provide the reference range of SIC of pregnant women in iodine‐sufficiency area. Methods Pregnant women were enrolled in the Department of Obstetrics, Tanggu Maternity Hospital, Tianjin from March 2016 to May 2017. Fasting venous blood and spot urine samples were collected. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyroid‐stimulating hormone (TSH), thyroglobulin (Tg), thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), UIC and SIC were measured. Results One thousand and ninety‐nine participants were included in this study. The median UIC was 156 μg/L. The median SIC was 108 μg/L, and the 95% reference interval for SIC was 65.6‐164.7 μg/L. SIC was positively correlated with UIC (r = 0.12, P < 0.001), FT3 (r = 0.23, P < 0.001), and FT4 (r = 0.50, P < 0.001) and was inversely correlated with TSH (r = −0.14, P < 0.001). Pregnant women with a SIC < 79.9 μg/L had a higher risk of hypothyroxinemia compared to those with higher SIC (OR = 2.44, 95% CI: 1.31‐4.75). Those having SIC > 138.5 μg/L were more likely to have thyrotoxicosis than those with lower SIC values (OR = 13.52, 95% CI: 4.21‐43.36). Conclusions Serum iodine level is associated with UIC and thyroid function in pregnant women. Low SIC was associated with increased risk for iodine deficiency and hypothyroxinemia, while high SIC was related to excess and thyrotoxicosis.

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

“…Spot UIC at the same time points also showed no obvious change rule (P > 0•05). The variation in spot UIC at fasting was different from the result of Pan (19) ; however, the variation in UI/Cr was consistent with Pan. Theoretically, with the increase in gestational weeks, morning sickness gradually subsides, and the total dietary energy intake and renal clearance of iodine increase, all of which caused 24-h UIE to show a gradually increasing trend (20) .…”

Urinary iodine level assessment during third trimester in a sample of Egyptian pregnant women and its relation to thyroid function

“…The results of our study was agreed with a Chinese study which described that pregnant women with a low iodine concentration during were 2.4 times more likely to develop hypothyroxinemia vs. women with a higher iodine concentration, whereas a high iodine concentration during pregnancy was related to risk for thyrotoxicosis. 17 Similarly, an Iranian study conducted on 203 pregnant women at third trimester, assessing UIC and thyroid function, categorized according to UIC into 2 groups:groups I and II (UIC <150 μg/liter and UIC >150 μg/liter respectively), showed a statistical significant difference between studied groups as regard TSH, as group I had the higher level of TSH and group II had the lower level of TSH. 14 Interestingly, inconsistent results were reported from studies investigating the correlation between median UIC in pregnant females and maternal thyroid function.…”

The Joint Role of Thyroid Function and Iodine Status on Risk of Preterm Birth and Small for Gestational Age: A Population-Based Nested Case-Control Study of Finnish Women

“…Furthermore, our study is the first to evaluate associations of preterm birth and SGA with serum iodine, a measure of iodine hypothesized to be more stable than urinary iodine concentrations over time. Single spot urinary iodine, which was used to assess iodine status in all previous studies on this subject, exhibits considerable intraindividual variation owing to urine dilution, dietary intake, circadian rhythm, season, and other factors [14,15,16]. Because of this, UIC is considered an acceptable biomarker of iodine status for whole populations, but not for individuals [35].…”

“…Several of these studies also lacked information on thyroid hormones and therefore were unable to evaluate whether associations of iodine with preterm birth and SGA may be different among potentially hypo-or hyperthyroid women [9,10,11,13]. Additionally, prior studies used a single spot urinary iodine measurement to classify iodine status, which has been shown to have high intraindividual variability reflective of recent dietary intake, seasonal variation, urine dilution, and circadian rhythmicity [13,14,15,16]. Although urine iodine concentrations are useful for assessing the iodine status of whole populations, serum iodide may be less sensitive to recent dietary intake and may, therefore, better reflect individual long-term and bioavailable iodine status [17,18,19], reducing potential for misclassification.…”