Use Of Iodine-containing Supplements Research Articles

Inadequate Iodine Intake in Mothers of Young Children in Innlandet County, Norway

BackgroundIodine has an essential role in child growth and brain development. Thus, sufficient iodine intake is particularly important in women of childbearing age and lactating women. ObjectivesThis cross-sectional study aimed to describe iodine intake in a large random sample of mothers of young children (aged ≤2 y) living in Innlandet County, Norway. MethodsFrom November 2020 to October 2021, 355 mother–child pairs were recruited from public health care centers. Dietary data were obtained using two 24-h dietary recalls (24-HRs) per woman and an electronic FFQ. The Multiple Source Method was used to estimate the usual iodine intake from the 24-HR assessment. ResultsBased on the 24-HRs, the median (P25, P75) usual iodine intake from food was 117 μg/d (88, 153) in nonlactating women and 129 μg/d (95, 176) in lactating women. The median (P25, P75) total usual iodine intake (from food combined with supplements) was 141 μg/d (97, 185) in nonlactating women and 153 μg/d (107, 227) in lactating women. Based on the 24-HRs, 62% of the women had a total iodine intake below the recommendations (150 μg/d in nonlactating women and 200 μg/d in lactating women), and 23% of them had an iodine intake below the average requirement (100 μg/d). The reported use of iodine-containing supplements was 21.4% in nonlactating women and 28.9% in lactating women. In regular users of iodine-containing supplements (n = 63), supplements contributed to an average of 172 μg/d of iodine. Among regular iodine supplement users, 81% reached the recommendations compared with 26% of nonsupplement users (n = 237). The iodine intake estimated by FFQ was substantially higher than that estimated by 24-HRs. ConclusionsMaternal iodine intake in Innlandet County was inadequate. This study confirms the need for action to improve iodine intake in Norway, particularly among women of childbearing age.

Iodine status in Danish pregnant women after an increase in iodine fortification.

Iodine fortification programmes are implemented in many countries and often associated with an increase in population iodine intake. However, the initial attempt may not be sufficient and in Denmark the level of iodine added to salt was increased in 2019. Sparse evidence is available on the impact of such modification in iodine fortification. The aim of this study was to evaluate iodine status in Danish pregnant women in 2021 after this increase in iodine fortification and compare to iodine status in 2012. Cross-sectional study. Pregnant women in the North Denmark Region referred for routine obstetric ultrasound in 2021. Participants filled out a questionnaire and delivered a spot urine. Median urinary iodine concentration (UIC) was calculated and assessed according to the recommended range in pregnancy (150-249 µg/L). Altogether 147 pregnant women were included and 88% used iodine-containing supplements. Median UIC was overall 77 µg/L [95% confidence interval (CI): 61-96 µg/L], which was lower than in 2012 (101 µg/L [95% CI: 89-111 µg/L]) (p < 0.001). Considering sources of iodine intake in pregnancy, lower daily intake of dairy products (p = 0.008) and bread (p < 0.001) and a lower content of iodine in the supplement used (p < 0.001) was seen in 2021 compared to 2012. Despite an increase in iodine fortification and frequent use of iodine-containing supplements, iodine status in pregnant women in the North Denmark Region was insufficient. Results call for continued monitoring and attention to ensure adequate iodine status during pregnancy in Denmark.

Iodine status of pregnant women from the Republic of Cyprus.

Iodine supply is crucial during pregnancy to ensure that the proper thyroid function of mother and baby support fetal brain development. Little is known about iodine status or its dietary determinants in pregnant women in the Republic of Cyprus. We therefore recruited 128 pregnant women at their first-trimester ultrasound scan to a cross-sectional study. We collected spot-urine samples for the measurement of urinary iodine concentration (UIC, µg/l) and creatinine concentration (Creat, g/l), the latter of which allows us to correct for urine dilution and to compute the iodine-to-creatinine ratio (UI/Creat). Women completed a FFQ and a general questionnaire. We used a General Linear model to explore associations between maternal and dietary characteristics with UI/Creat. The median UIC (105 µg/l) indicated iodine deficiency according to the WHO criterion (threshold for adequacy = 150 µg/l), and the UI/Creat was also low at 107 µg/g. Only 32 % (n 45) of women reported the use of iodine-containing supplements; users had a higher UI/Creat than non-users (131 µg/g v. 118 µg/g), though this difference was NS in the adjusted analysis (P = 0·37). Of the dietary components, only egg intake was significantly associated with a higher UI/Creat in adjusted analyses (P = 0·018); there was no significant association with milk, dairy products or fish intake. Our results suggest that pregnant women in Cyprus have inadequate iodine status and are at risk of mild-to-moderate iodine deficiency. Further research on dietary sources in this population is required.

Iodine status of postpartum women and their infants aged 3, 6 and 12 months: Mother and Infant Nutrition Investigation (MINI).

To alleviate the re-emergence of iodine deficiency in New Zealand, two strategies, the mandatory fortification of bread with iodised salt (2009) and a government-subsidised iodine supplement for breast-feeding women (2010), were introduced. Few studies have investigated mother and infant iodine status during the first postpartum year; this study aimed to describe iodine status of mothers and infants at 3, 6 and 12 months postpartum (3MPP, 6MPP and 12MPP, respectively). Partitioning of iodine excretion between urine and breast milk of exclusive breast-feeding (EBF) women at 3MPP was determined. In total, eighty-seven mother-infant pairs participated in the study. Maternal and infant spot urinary iodine concentration (UIC) and breast milk iodine concentration (BMIC) were determined. The percentage of women who took iodine-containing supplements decreased from 46 % at 3MPP to 6 % at 12MPP. Maternal median UIC (MUIC) at 3MPP (82 (46,157) µg/l), 6MPP (85 (43,134) µg/l) and 12MPP (95 (51,169) µg/l) were <100 µg/l. The use of iodine-containing supplements increased MUIC and BMIC only at 3MPP. Median BMIC at all time points were below 75 µg/l. Infant MUIC at 3MPP (115 (69,182) µg/l) and 6MPP (120 (60,196) µg/l) were below 125 µg/l. Among EBF women at 3MPP, an increased partitioning of iodine into breast milk (highest proportion 60 %) was shown at lower iodine intakes, along with a reduced fractional iodine excretion in urine (lowest proportion 40 %), indicating a protective mechanism for breastfed infants' iodine status. In conclusion, this cohort of postpartum women was iodine-deficient. Iodine status of their breastfed infants was suboptimal. Lactating women who do not consume iodine-rich foods and those who become pregnant again should take iodine-containing supplements.

Iodine status during pregnancy and at 6 weeks, 6, 12 and 18 months post-partum.

Iodine deficiency during pregnancy and in the post‐partum period may lead to impaired child development. Our aim is to describe iodine status longitudinally in women from pregnancy until 18 months post‐partum. Furthermore, we explore whether iodine status is associated with dietary intake, iodine‐containing supplement use and breastfeeding status from pregnancy until 18 months post‐partum. We also assess the correlation between maternal iodine status 18 months post‐partum and child iodine status at 18 months of age. Iodine status was measured by urinary iodine concentration (UIC) during pregnancy (n = 1,004), 6 weeks post‐partum (n = 915), 6 months post‐partum (n = 849), 12 months post‐partum (n = 733) and 18 months post‐partum (n = 714). The toddlers' UIC was assessed at 18 months of age (n = 416). Demographic variables and dietary data (food frequency questionnaire) were collected during pregnancy, and dietary data and breastfeeding practices were collected at all time points post‐partum. We found that iodine status was insufficient in both pregnant and post‐partum women. The UIC was at its lowermost 6 weeks post‐partum and gradually improved with increasing time post‐partum. Intake of milk and use of iodine‐containing supplements significantly increased the odds of having a UIC above 100 μg/L. Neither the mothers' UIC, vegetarian practice, nor exclusion of milk and dairy products were associated with the toddlers UIC 18 months post‐partum. Women who exclude milk and dairy products from their diets and/or do not use iodine‐containing supplements may be at risk of iodine deficiency. The women possibly also have an increased risk of thyroid dysfunction and for conceiving children with nonoptimal developmental status.

Iodine status in breastfeeding women and their infants in New Zealand

AbstractIodine is an essential micronutrient in thyroid hormone synthesis and physical and mental development. Iodine deficiency has historically been a problem in New Zealand. Two government initiatives were introduced: 1) mandatory fortification of all bread (except organic) with iodised salt (2009); 2) provision of a subsidised iodine supplement for all pregnant and breastfeeding women (2010). Since these initiatives, the majority of adults and children have achieved adequate iodine status. However, iodine status among breastfeeding women and their infants is unknown. This observational follow-up cohort study recruited 87 breastfeeding women, aged 16 years and over, with a healthy term singleton infant, in the Manawatu region, New Zealand. Women were recruited at 3 months postpartum; and followed up at 6 and 12 months postpartum. A maternal questionnaire investigated supplement use before and after delivery, iodine nutritional knowledge, and demographic information. Maternal spot urine, breastmilk, and infant spot urine samples were collected and iodine concentrations determined using inductively-coupled plasma mass spectrometry. Iodine-containing supplement use was recorded in the 24 hours before biological sample collection. Shapiro-Wilk test was used to check the normality of the data. Iodine data were analysed using non-parametric Mann Whitney U test (2-tailed).Thirty-five (40%) women used iodine-containing supplements during breastfeeding. Median urine iodine concentration (UIC) was 82 (46, 157) μg/L, below the WHO cut-off value of 100 μg/L for breastfeeding women. Median infant UIC was 115 (69, 182) μg/L, above the WHO cut-off value of 100 μg/L. Median maternal UIC was significantly higher for supplement users than non-users (111(44, 240) vs 62(42, 102) μg/L; p = 0.021]. Median infant UIC was significantly higher for supplement users than non-users (150(97, 193) vs 86(53, 171) μg/L; p = 0.045]. Median breastmilk iodine concentration (BMIC) was 69 (52, 119) μg/L, lower than recommended (75 μg/L). Median BMIC was significantly higher for supplement users than non-users [83(67, 168) vs 62(48, 82) μg/L; p = 0.000]. Overall, iodine status within this sample of breastfeeding women remains inadequate at 3 months postpartum. Users of iodine containing supplements and their infants achieved recommended iodine UIC concentrations; however, non-users and their infants had inadequate status. Despite recommendations that all breastfeeding women use iodine supplements, supplement use was low (only 40%). Further work is require to investigate if current iodine status affects thyroid function. Alternative strategies are also required to ensure breastfeeding women and their infants have adequate iodine.

Inadequate iodine intake is associated with subfecundity in mild-to-moderately iodine deficient Norwegian women

AbstractIodine is an essential micronutrient and an integral part of the thyroid hormones. In women of childbearing age, the estimated average iodine requirement is 95 μg/day and the recommended daily intake is 150 μg/day. While severe iodine deficiency poses reproductive risks, including infertility and abortions, the potential impact of mild-to-moderate iodine deficiency on subfecundity is unknown.We examined whether iodine intake was associated with risk of subfecundity (i.e. &gt; 12 months trying to get pregnant) in a large cohort of mild-to-moderately iodine deficient women.Women enrolled in the Norwegian Mother and Child Cohort Study in gestational week 15 were asked to report whether the pregnancy was planned and how many months the couple had sexual relations without any contraception before getting pregnant. Information about time to pregnancy, maternal characteristics and iodine intake was available for 56,416 planned pregnancies. The median (interquartile range) time to pregnancy was 1.5 (0.5–6.0) months and the prevalence of subfecundity was 10.8%). We used iodine intake assessed by a validated food frequency questionnaire administered in pregnancy as a proxy for long-term (pre-pregnancy) iodine intake. We used logistic regression to estimate the association between iodine intake and subfecundity, using flexible modelling with restricted cubic splines, and adjusted for maternal age, BMI, parity, education, smoking status, energy intake and fiber intake. The median calculated iodine intake was 121 μg/day and the median urinary iodine concentration in a subsample of n = 2795 women was 69 μg/L.The prevalence of subfecundity was lowest for iodine intakes ~100 μg/day and increased at lower intakes (p overall = 0.005). Compared to an intake of 100 μg/day (reference), intakes ~75 μg/day was associated with 5% (95%CI: 1%, 9%) higher prevalence and intakes ~50 μg/day with 14% (95%CI: 4%, 26%) higher prevalence. Use of dietary supplements was recorded only for the last 6 months prior to conception and women were included in the analysis regardless of their reported supplement use. In a sensitivity analysis, we excluding women who reported iodine-containing supplement use in the period 26–9 weeks before conception and the result remained unchanged. We also modelled time to pregnancy by Cox regression, and the result was consistent with the result for subfecundity.The only good dietary sources of iodine in Norway are milk and white fish, and many women of fertile age have low intakes of these food items. This study shows that low habitual iodine intake may be a risk factor for subfecundity.

Iodine Intake is Associated with Thyroid Function in Mild to Moderately Iodine Deficient Pregnant Women.

Background: Studies indicate that mild to moderate iodine deficiency in pregnancy may have a long-term negative impact on child neurodevelopment. These effects are likely mediated via changes in maternal thyroid function, since iodine is essential for the production of thyroid hormones. However, the impact of iodine availability on thyroid function during pregnancy and on thyroid function reference ranges are understudied. The aim of this study was to investigate the association between iodine intake and thyroid function during pregnancy.Design: In a population-based pregnancy cohort including 2910 pregnant women participating in The Norwegian Mother and Child Cohort Study, we explored cross sectional associations of maternal iodine intake measured (1) by a food frequency questionnaire and (2) as iodine concentration in a spot urine sample, with plasma thyroid hormones and antibodies.Results: Biological samples were collected in mean gestational week 18.5 (standard deviation 1.3) and diet was assessed in gestational week 22. Median iodine intake from food was 121 μg/day (interquartile range 90, 160), and 40% reported use of iodine-containing supplements in pregnancy. Median urinary iodine concentration (UIC) was 59 μg/L among those who did not use supplements and 98 μg/L in the women reporting current use at the time of sampling, indicating mild to moderate iodine deficiency in both groups. Iodine intake as measured by the food frequency questionnaire was not associated with the outcome measures, while UIC was inversely associated with FT3 (p = 0.002) and FT4 (p < 0.001). Introduction of an iodine-containing supplement after gestational week 12 was associated with indications of lower thyroid hormone production (lower FT4, p = 0.027, and nonsignificantly lower FT3, p = 0.17). The 2.5th and 97.5th percentiles of TSH, FT4, and FT3 were not significantly different by groups defined by calculated iodine intake or by UIC.Conclusion: The results indicate that mild to moderate iodine deficiency affect thyroid function in pregnancy. However, the differences were small, suggesting that normal reference ranges can be determined based on data also from mildly iodine deficient populations, but this needs to be further studied. Introducing an iodine-containing supplement might temporarily inhibit thyroid hormone production and/or release.

Maternal Iodine Status is Associated with Offspring Language Skills in Infancy and Toddlerhood.

Inadequate iodine status affects the synthesis of the thyroid hormones and may impair brain development in fetal life. The aim of this study was to explore the association between maternal iodine status in pregnancy measured by urinary iodine concentration (UIC) and child neurodevelopment at age 6, 12 and 18 months in a population-based cohort. In total, 1036 families from nine locations in Norway were enrolled in the little in Norway cohort. The present study includes n = 851 mother-child pairs with singleton pregnancies, no use of thyroid medication in pregnancy, no severe genetic disorder, data on exposure (UIC) in pregnancy and developmental outcomes (Bayley Scales of Infant and Toddler Development, third edition). Data collection also included general information from questionnaires. We examined associations between UIC (and use of iodine-containing supplements) and repeated measures of developmental outcomes using multivariable mixed models. The median UIC in pregnancy was 78 µg/L (IQR 46–130), classified as insufficient iodine intake according to the WHO. Eighteen percent reported use of iodine-containing multisupplements. A UIC below ~100 was associated with reduced receptive (p = 0.025) and expressive language skills (p = 0.002), but not with reduced cognitive or fine- and gross motor skills. Maternal use of iodine-containing supplements was associated with lower gross motor skills (b = −0.18, 95% CI = −0.33, −0.03, p = 0.02), but not with the other outcome measures. In conclusion, an insufficient iodine intake in pregnancy, reflected in a UIC below ~100 µg/L, was associated with lower infant language skills up to 18 months. The use of iodine-containing supplements was not associated with beneficial effects.

Iodine-containing supplement use by pregnant women attending antenatal clinics in Western Australia.

Iodine requirements increase during pregnancy and although national recommendations for daily iodine supplementation (150µg) exist, there is no research related to the uptake of these recommendations by pregnant women in Western Australia. To investigate the use of iodine-containing supplements and associations with their use in a sample of Western Australian pregnant women. A cross-sectional study was conducted on pregnant women attending antenatal clinics in a public tertiary hospital for women and neonates in Perth during 2012 and 2013 (n=425). Women completed a self-administered questionnaire. Frequencies and percentages were obtained for categorical variables and χ2 tests conducted to assess associations between iodine-containing supplement use and sociodemographic and pregnancy-related factors. A total of 24% of pregnant women reported using iodine-containing supplements prior to pregnancy compared to 66% during the previous two months. Age and maternal income were associated with use prior to pregnancy only (P=0.004 and P=0.031) and first pregnancy was associated with use during pregnancy only (P=0.006). Ethnicity and reporting use in the first two trimesters were associated with the use of iodine supplements both in the year prior to pregnancy (P=0.002 and P=0.020, respectively) and during pregnancy (P<0.001 and P=0.001, respectively). Two-thirds of women reported use of iodine-containing supplements during pregnancy, within the range reported for other Australian states. One-quarter reported use prior to pregnancy. Public health strategies are required to promote awareness of the importance of iodine and supplementation both before and during the entire pregnancy.

Maternal Iodine Intake and Offspring Attention-Deficit/Hyperactivity Disorder: Results from a Large Prospective Cohort Study.

Current knowledge about the relationship between mild to moderately inadequate maternal iodine intake and/or supplemental iodine on child neurodevelopment is sparse. Using information from 77,164 mother-child pairs in the Norwegian Mother and Child Cohort Study, this study explored associations between maternal iodine intake and child attention-deficit/hyperactivity disorder (ADHD) diagnosis, registered in the Norwegian Patient Registry and maternally-reported child ADHD symptoms at eight years of age. Pregnant women reported food and supplement intakes by questionnaire in gestational week 22. In total, 1725 children (2.2%) were diagnosed with ADHD. In non-users of supplemental iodine (53,360 mothers), we found no association between iodine intake from food and risk of child ADHD diagnosis (p = 0.89), while low iodine from food (<200 µg/day) was associated with higher child ADHD symptom scores (adjusted difference in score up to 0.08 standard deviation (SD), p < 0.001, n = 19,086). In the total sample, we found no evidence of beneficial effects of maternal use of iodine-containing supplements (n = 23,804) on child ADHD diagnosis or symptom score. Initiation of iodine supplement use in gestational weeks 0–12 was associated with an increased risk of child ADHD (both measures). In conclusion, insufficient maternal iodine intake was associated with increased child ADHD symptom scores at eight years of age, but not with ADHD diagnosis. No reduction of risk was associated with maternal iodine supplement use.

Assessment of Breast Milk Iodine Concentrations in Lactating Women in Western Australia.

Breast-fed infants may depend solely on an adequate supply of iodine in breast milk for the synthesis of thyroid hormones which are essential for optimal growth and cognitive development. This is the first study to measure breast milk iodine concentration (BMIC) among lactating women in Western Australian (n = 55). Breast milk samples were collected between 2014 and 2015 at a mean (±SD) of 38.5 (±5.5) days post-partum. The samples were analysed to determine median BMIC and the percentage of samples with a BMIC < 100 µg/L, a level considered adequate for breast-fed infants. The influence of (a) iodine-containing supplements and iodised salt use and (b) consumption of key iodine-containing foods on BMIC was also examined. The median (p25, p75) BMIC was 167 (99, 248) µg/L and 26% of samples had a BMIC < 100 µg/L. Overall, BMIC tended to be higher with iodine-containing supplement usage (ratio 1.33, 95% confidence interval (CI) (1.04, 1.70), p = 0.030), cow’s milk consumption (ratio 1.66, 95% CI (1.23, 2.23), p = 0.002) and lower for Caucasians (ratio 0.61, 95% CI (0.45, 0.83), p = 0.002), and those with secondary school only education (ratio 0.66, 95% CI (0.46, 0.96), p = 0.030). For most women, BMIC was adequate to meet the iodine requirements of their breast-fed infants. However, some women may require the use of iodine-containing supplements or iodised salt to increase BMIC to adequate levels for optimal infant nutrition.

Food Group Intakes as Determinants of Iodine Status among US Adult Population

Adequate intake of iodine is essential for proper thyroid function. Although dietary reference intakes for iodine have been established, iodine intake cannot be estimated due to the lack of data on iodine contents in foods. We aimed to determine if food group intakes can predict iodine status assessed by urinary iodine concentration (UIC) from spot urine samples of 5967 US adults in the National Health and Nutrition Examination Survey (NHANES) 2007–2012. From an in-person 24-h dietary recall, all foods consumed were aggregated into 12 main food groups using the individual food code of the US Department of Agriculture (USDA); dairy products, meat/poultry, fish/seaweed, eggs, legumes/nuts/seeds, breads, other grain products, fruits, vegetables, fats/oils, sugars/sweets, and beverages. Chi-square test, Spearman correlation, and multiple linear regression analyses were conducted to investigate the predictability of food group intakes in iodine status assessed by UIC. From the multiple linear regressions, the consumption of dairy products, eggs, and breads, and iodine-containing supplement use were positively associated with UIC, whereas beverage consumption was negatively associated with UIC. Among various food group intakes, dairy product intake was the most important determinant of iodine status in both US men and women. Subpopulation groups with a high risk of iodine deficiency may need nutritional education regarding the consumption of dairy products, eggs, and breads to maintain an adequate iodine status. Efforts toward a better understanding of iodine content in each food and a continued monitoring of iodine status within US adults are both warranted.

Risk of Suboptimal Iodine Intake in Pregnant Norwegian Women

Pregnant women and infants are exceptionally vulnerable to iodine deficiency. The aims of the present study were to estimate iodine intake, to investigate sources of iodine, to identify predictors of low or suboptimal iodine intake (defined as intakes below 100 μg/day and 150 μg/day) in a large population of pregnant Norwegian women and to evaluate iodine status in a sub-population. Iodine intake was calculated based on a validated Food Frequency Questionnaire in the Norwegian Mother and Child Cohort. The median iodine intake was 141 μg/day from food and 166 μg/day from food and supplements. Use of iodine-containing supplements was reported by 31.6%. The main source of iodine from food was dairy products, contributing 67% and 43% in non-supplement and iodine-supplement users, respectively. Of 61,904 women, 16.1% had iodine intake below 100 μg/day, 42.0% had iodine intake below 150 μg/day and only 21.7% reached the WHO/UNICEF/ICCIDD recommendation of 250 μg/day. Dietary behaviors associated with increased risk of low and suboptimal iodine intake were: no use of iodine-containing supplements and low intake of milk/yogurt, seafood and eggs. The median urinary iodine concentration measured in 119 participants (69 μg/L) confirmed insufficient iodine intake. Public health strategies are needed to improve and secure the iodine status of pregnant women in Norway.

Maternal thyroid function in different conditions of iodine nutrition in pregnant women exposed to mild-moderate iodine deficiency: an observational study

We examined the effect of different conditions of nutritional iodine intake on maternal thyroid function throughout gestation in a cohort of healthy, anti-thyroid antibody-negative women from a mild-moderately iodine-deficient (ID) area. Observational cohort study. The study included 168 women receiving prenatal preparations containing 150 μg of iodine from early pregnancy (150-I group); 105 women who had regularly used (>2 years) iodized salt prior to becoming pregnant (I-salt group); 160 women neither taking iodine supplements nor using iodized salt (no-I group). Maternal TSH, FT3 and FT4 were determined throughout gestation. Mean TSH concentrations were higher among the 150-I women than in the remaining two groups, and in a high proportion of them, TSH values were found to exceed the upper limit for gestational age. Conversely, the prevalence of low free-thyroxine levels in the 150-I women was similar to that observed in the I-salt women and markedly lower than that recorded for the no-I group. The regular use of iodine-containing supplements proved effective in reducing the risk of inappropriately low FT4 levels during pregnancy. The observed TSH increase in 150-I women may be because of a transient stunning effect on the thyroid gland, occurring as a result of the abrupt increase in daily iodine intake. Whilst the importance of gestational iodine supplementation is undisputed, we believe that in mild-moderately ID areas, women considering conception should be advised to take iodine supplementation for several months prior to pregnancy.

Naturally occurring iodine in humic substances in drinking water in Denmark is bioavailable and determines population iodine intake

Iodine intake is important for thyroid function. Iodine content of natural waters is high in some areas and occurs bound in humic substances. Tap water is a major dietary source but bioavailability of organically bound iodine may be impaired. The objective was to assess if naturally occurring iodine bound in humic substances is bioavailable. Tap water was collected at Randers and Skagen waterworks and spot urine samples were collected from 430 long-term Randers and Skagen dwellers, who filled in a questionnaire. Tap water contained 2 microg/l elemental iodine in Randers and 140 microg/l iodine bound in humic substances in Skagen. Median (25; 75 percentile) urinary iodine excretion among Randers and Skagen dwellers not using iodine-containing supplements was 50 (37; 83) microg/24 h and 177 (137; 219) microg/24 h respectively (P < 0.001). The fraction of samples with iodine below 100 microg/24 h was 85.0 % in Randers and 6.5 % in Skagen (P < 0.001). Use of iodine-containing supplements increased urinary iodine by 60 microg/24 h (P < 0.001). This decreased the number of samples with iodine below 100 microg/24 h to 67.3 % and 5.0 % respectively, but increased the number of samples with iodine above 300 microg/24 h to 2.4 % and 16.1 %. Bioavailability of iodine in humic substances in Skagen tap water was about 85 %. Iodine in natural waters may be elemental or found in humic substances. The fraction available suggests an importance of drinking water supply for population iodine intake, although this may not be adequate to estimate population iodine intake.