title: Gestational changes in iodine status in a cohort study of pregnant women from the United Kingdom: season as an effect modifier.
abstract: 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.
  
  
  OBJECTIVE
  The aims were to evaluate iodine status in a cohort of UK pregnant women and to explore how it changes throughout gestation.
  
  
  METHODS
  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 ∼12, 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.
authors.affiliation: From the University of Surrey, Guildford, United Kingdom (SCB, VF-O, and MPR); Danone Nutricia Early Life Nutrition, Trowbridge, United Kingdom (VF-O); and the Nuffield Department of Obstetrics and Gynecology, University of Oxford, Oxford, United Kingdom (CWGR).
authors.forename: Sarah C
authors.initials: SC
authors.lastname: Bath
authors.affiliation: From the University of Surrey, Guildford, United Kingdom (SCB, VF-O, and MPR); Danone Nutricia Early Life Nutrition, Trowbridge, United Kingdom (VF-O); and the Nuffield Department of Obstetrics and Gynecology, University of Oxford, Oxford, United Kingdom (CWGR).
authors.forename: Victoria L
authors.initials: VL
authors.lastname: Furmidge-Owen
authors.affiliation: From the University of Surrey, Guildford, United Kingdom (SCB, VF-O, and MPR); Danone Nutricia Early Life Nutrition, Trowbridge, United Kingdom (VF-O); and the Nuffield Department of Obstetrics and Gynecology, University of Oxford, Oxford, United Kingdom (CWGR).
authors.forename: Christopher Wg
authors.initials: CW
authors.lastname: Redman
authors.affiliation: From the University of Surrey, Guildford, United Kingdom (SCB, VF-O, and MPR); Danone Nutricia Early Life Nutrition, Trowbridge, United Kingdom (VF-O); and the Nuffield Department of Obstetrics and Gynecology, University of Oxford, Oxford, United Kingdom (CWGR). m.rayman@surrey.ac.uk.
authors.forename: Margaret P
authors.initials: MP
authors.lastname: Rayman
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D003404:Creatinine / Q000652:urine; D003430:Cross-Sectional Studies; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D007455:Iodine / Q000172:deficiency / Q000652:urine*; D016014:Linear Models; D039382:Maternal Nutritional Physiological Phenomena; D009752:Nutritional Status; D011247:Pregnancy; D011264:Pregnancy Trimesters; D016032:Randomized Controlled Trials as Topic; D012621:Seasons; D011795:Surveys and Questionnaires; D013963:Thyroid Hormones / Q000378:metabolism; D006113:United Kingdom; D055815:Young Adult
chemical_list: D013963:Thyroid Hormones; D007455:Iodine; D003404:Creatinine
keywords: United Kingdom; deficiency; diet; iodine; pregnancy