ABSTRACT
BACKGROUND: Though iodine deficiency in pregnancy is a matter of public-health concern, a functional measure of iodine status is lacking. The thyroid-specific protein thyroglobulin (Tg), which reflects thyroid size, has shown promise as a functional measure in studies of children and adults, but data in pregnancy are sparse. In a cohort of mildly to moderately iodine-deficient pregnant women, this study aimed to explore whether serum Tg is a sensitive functional biomarker of iodine status and to examine longitudinal change in Tg with gestational age. METHOD: A total of 230 pregnant women were recruited at an antenatal clinic at 12 weeks of gestation to the Selenium in PRegnancy INTervention study, in Oxford, United Kingdom. Repeated measures of urinary iodine-to-creatinine ratio, serum thyrotropin (TSH), and Tg at 12, 20, and 35 weeks of gestation were made. Women were dichotomized by their iodine-to-creatinine ratio (<150 or ≥150 µg/g) to group them broadly as iodine deficient or iodine sufficient. Women with thyroid antibodies were excluded; data and samples were available for 191 women. RESULTS: Median Tg concentrations were 21, 19, and 23 µg/L in the first, second, and third trimesters, respectively. In a linear mixed model, controlling for confounders, Tg was higher in the <150 µg/g group than it was in the ≥150 µg/g group (p < 0.001) but there was no difference in TSH (p = 0.27). Gestational week modified the effect of iodine status on TSH (p = 0.01) and Tg (p = 0.012); Tg did not increase with gestational week in the ≥150 µg/g group, but it did in the <150 µg/g group, and TSH increased more steeply in the <150 µg/g group. CONCLUSIONS: Low iodine status (<150 µg/g) in pregnancy is associated with higher serum Tg, suggesting that the thyroid is hyperstimulated by iodine deficiency, which causes it to enlarge. Tg is a more sensitive biomarker of iodine status in pregnancy than is TSH.
Subject(s)
Deficiency Diseases/blood , Iodine/deficiency , Pregnancy Complications/blood , Thyroglobulin/blood , Adult , Biomarkers/blood , Creatinine/urine , Deficiency Diseases/diagnosis , Female , Humans , Iodine/urine , Linear Models , Pregnancy , Pregnancy Complications/diagnosis , Pregnancy Trimester, First , Pregnancy Trimester, Second , Pregnancy Trimester, Third , Sensitivity and Specificity , Thyrotropin/blood , United KingdomABSTRACT
BACKGROUND: Iodine is required throughout pregnancy for thyroid hormone production, which is essential for fetal brain development. Studies of iodine status in pregnant women from the United Kingdom (UK) have focused on early gestation (<16 wk). Data on the effect of advancing gestation on urinary iodine excretion are conflicting, with suggestions of both an increase and a decrease. OBJECTIVES: The aims were to evaluate iodine status in a cohort of UK pregnant women and to explore how it changes throughout gestation. DESIGN: We used samples and data from 230 UK pregnant women who were recruited to the Selenium in PRegnancy INTervention study. Iodine concentration was measured in spot-urine samples that were collected at â¼12, 20, and 35 wk of gestation; creatinine concentration was also measured to correct for urine dilution. A linear mixed model was used to explore the effect of gestational week on iodine-to-creatinine ratio, with change in season, body mass index, daily milk intake, and maternal age controlled for. RESULTS: The median urinary iodine concentration from urine samples collected at all time points (n = 662) was 56.8 µg/L, and the iodine-to-creatinine ratio was 116 µg/g, thus classifying this cohort as mildly-to-moderately iodine deficient. The median iodine-to-creatinine ratios at 12, 20, and 35 wk were 102.5, 120.0, and 126.0 µg/g, respectively. Only 3% of women were taking iodine-containing prenatal supplements. The iodine-to-creatinine ratio increased with advancing gestation, and there was a significant interaction between gestational week and season (P = 0.026). For a 1-wk increase in gestation, the iodine-to-creatinine ratio increased by a factor of 1.05 (95% CI: 1.02, 1.08) in winter and by a factor of 1.04 (95% CI: 1.00, 1.08) in summer. CONCLUSIONS: This group of UK pregnant women was mildly-to-moderately iodine deficient at all trimesters, which is of public health concern. The finding that the iodine-to-creatinine ratio increased over the course of gestation may not be generalizable to populations with different iodine status from ours and merits further investigation. This trial was registered at www.isrctn.com as ISRCTN37927591.
