Urinary iodine
Urinary iodine reflects iodine intake over recent days. WHO classes a population median of 100 to 199 µg/L as adequate, or 150 to 249 in pregnancy, and states the test is not useful for diagnosing one person. A single sample varied about 38% within a person. Populations with excess iodine had more hypothyroidism.
- Status
- A urine test ordered by a clinician, run on a single urine sample. WHO describes it as a marker of recent iodine intake for populations and states that urinary iodine concentrations are not useful for the diagnosis and treatment of individuals [1].
- Measures
- The concentration of iodine in a single urine sample, which reflects iodine intake over the previous days because more than 90% of dietary iodine appears in the urine [2].
- Also called
- urinary iodine concentration, UIC, urine iodine, iodine urine, spot urine iodine, random urine iodine, iodine random urine, urinary iodide
- Tested for
- iodine deficiency, iodine status, thyroid health, goiter, pregnancy planning, checking an iodine supplement, iodine excess
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Linked in research1
Reported risks0
No risks sourced yet.
Reference ranges
Each range is the one its source published, in µg/L. Also reported in nmol/L. Where sources differ, each is shown.
World Health Organization, 2013, adults
The median of a population of adults who are not pregnant or lactating · [1], [3]
| Range | What the source calls it |
|---|---|
| below 20 µg/L | "severe iodine deficiency" |
| 20 to 49 µg/L | "moderate iodine deficiency" |
| 50 to 99 µg/L | "mild iodine deficiency" |
| 100 to 199 µg/L | "adequate iodine nutrition" |
| 200 to 299 µg/L | "above requirements" |
| 300 or above µg/L | "excessive" |
WHO sets these categories for the median of a population, not for one person. It states that an individual's urinary iodine concentration can vary daily and even within a day, that the variation evens out across a population, and that the test is therefore not useful for the diagnosis and treatment of individuals. A single result placed here shows only where that one sample would sit on a scale built for groups. The cut-off values are based on school-age children, and WHO's table states that they apply to adults other than pregnant and lactating women. The first four labels are WHO's words for iodine status. For the two highest bands WHO's status column gives a description in place of a name, so the labels here are its words for iodine intake. WHO defines excessive as in excess of the amount required to prevent and control iodine deficiency [1]. WHO adds that not more than 20% of samples in a population are to be below 50 µg/L [3]. For lactating women WHO gives two categories, insufficient below 100 µg/L and adequate at 100 µg/L or above, with the same statement that they describe a population median [1]. The factor to nmol/L follows WHO's laboratory annex, which gives 1 µg of iodine per mL as 7.9 µmol/L [3].
World Health Organization, 2013, school-age children
The median of a population of school-age children aged 6 years or older · [1], [3]
| Range | What the source calls it |
|---|---|
| below 20 µg/L | "severe iodine deficiency" |
| 20 to 49 µg/L | "moderate iodine deficiency" |
| 50 to 99 µg/L | "mild iodine deficiency" |
| 100 to 199 µg/L | "adequate iodine nutrition" |
| 200 to 299 µg/L | "above requirements" |
| 300 or above µg/L | "excessive" |
WHO sets these categories for the median of a population of children, not for one child, and states that urinary iodine concentrations are not useful for the diagnosis and treatment of individuals. School-age children are the group the cut-off values were first built on. WHO describes 200 to 299 µg/L as a level that may pose a slight risk of more than adequate iodine intake, and 300 µg/L or above as carrying a risk of adverse health consequences, naming iodine-induced hyperthyroidism and autoimmune thyroid disease [1]. WHO's guide for program managers states that about 30 urine samples from a defined group are sufficient to allow for differences in hydration and other biological variation between individuals [3].
World Health Organization, 2013, children under 2 years
The median of a population of children younger than 2 years · [1]
| Range | What the source calls it |
|---|---|
| below 100 µg/L | "insufficient" |
| 100 or above µg/L | "adequate" |
WHO sets these categories for the median of a population, not for one child, and states that urinary iodine concentrations are not useful for the diagnosis and treatment of individuals. WHO publishes only these two categories for children under 2, so this block does not classify high results [1].
World Health Organization, 2013, pregnancy
The median of a population of pregnant women · [1]
| Range | What the source calls it |
|---|---|
| below 150 µg/L | "insufficient" |
| 150 to 249 µg/L | "adequate" |
| 250 to 499 µg/L | "above requirements" |
| 500 or above µg/L | "excessive" |
WHO sets these categories for the median of a population of pregnant women, not for one woman, and states that urinary iodine concentrations are not useful for the diagnosis and treatment of individuals. The cut points are higher than for other adults because iodine requirements rise in pregnancy. The labels are WHO's words for iodine intake. Its table gives no separate grades of deficiency in pregnancy [1].
What has been shown to move it
Shown to lower it1
Iodine
One trial found urinary iodine fell by half within 6 months in postmenopausal women, while it held steady on a Nordic guideline diet.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- Median 24-hour urinary iodine concentration fell from 71.0 to 36.0 μg/L at 6 months (p 0.001), and 24-hour iodine excretion from 134.0 to 77.0 μg/d. Levels did not change on the comparison diet. FT4 and TSH did not differ, and FT3 was lower at 6 months [8]. In a 4-week trial, iodine intake was 47.9 μg a day lower than on the Australian national guide diet (95% CI 16.5 to 79.2) [7].
- Population
- 70 postmenopausal women with overweight or obesity over 2 years [8]; 39 women over 4 weeks [7].
- Sources
- [7], [8]
- Caveats
- The diet excludes table salt and dairy, which the authors call two of the largest iodine sources [8]. Small trials in women.
How it compares with the reference method
Matched the reference method1
Spot sample against a 24-hour collection
One US study found the median spot concentration did not differ from the 24-hour concentration in a group of 400 adults, while other iodine measures were not interchangeable.
The numbers and sources
- Compared with
- Iodine concentration in a complete 24-hour urine collection
- Effect
- Volunteers collected all urine for 24 hours. The median 24-hour concentration was 144.8 µg/L, and medians from timed spot samples ran from 147.3 to 156.2 µg/L, with no consistent difference by time of day. The median total excreted was 173.6 µg a day, and the median iodine to creatinine ratio in spot samples was 103.6 to 114.3 µg/g. The authors conclude that concentration, the creatinine ratio and estimated 24-hour excretion are not to be used interchangeably [11]. A European guideline gives a reference value of 100 to 300 µg per 24 hours for urinary iodine, which is an amount per day and is not placed against a result typed here [12]. In the US survey of 1988 to 1994, urinary iodine concentration in µg/L was 1.16 times the value expressed in µg per gram of creatinine [2].
- Population
- 400 volunteers aged 18 to 39 in the United States [11].
- Sources
- [11], [12], [2]
- Caveats
- The agreement is for the median of a group, not for one person's spot sample against that person's own 24-hour collection. A lab report in µg per 24 hours or µg per gram of creatinine is a different quantity from the µg/L on this page.
Did not match the reference method1
One sample against a person's usual level
Small studies found a spot result varied about 38% within one person, so 10 or more samples were needed to place an individual within 20%.
The numbers and sources
- Compared with
- The same person's results across repeated collections over 13 to 15 months
- Effect
- In women who collected repeated samples for 15 months, the within-person coefficient of variation was 38% for spot urinary iodine concentration and 32% for iodine measured in 24-hour collections, and 10 spot or 24-hour samples were needed to estimate one woman's iodine status with 20% precision [9]. In men sampled monthly for 13 months, single spot results ranged from 10 to 260 µg/L while each man's own average ranged from 30 to 87 µg/L. Twelve or more samples were needed for 20% precision in an individual, and the authors conclude that fewer than ten samples in an individual may be misleading [10].
- Population
- 22 women aged 52 to 77 in Switzerland, with 177 spot and 341 24-hour samples [9]. 16 men living in an area of mild to moderate iodine deficiency [10].
- Sources
- [9], [10]
- Caveats
- Two small studies, of 22 women and 16 men. Both conclude that a single sample does not establish an individual's iodine status, which matches WHO's statement that the test is for populations [1]. WHO attributes part of the variation between samples to hydration [3].
What studies tie it to
Linked in research1
Thyroid disorders in populations with high levels
Linked to more hypothyroidism in populations with excess iodine, about twice the odds of subclinical hypothyroidism in a meta-analysis of three studies.
The numbers and sources
- Effect
- In a meta-analysis of cross-sectional studies in adults, populations with excess iodine had higher odds of overt hypothyroidism (OR 2.78, 95% CI 1.47 to 5.27) and subclinical hypothyroidism (OR 2.03, 1.58 to 2.62) than populations with adequate iodine [7]. In a five-year study of three regions of China with median urinary iodine of 84, 243 and 651 µg/L, the cumulative incidence of subclinical hypothyroidism was 0.2%, 2.6% and 2.9%, and that of autoimmune thyroiditis was 0.2%, 1.0% and 1.3%. Overt hypothyroidism developed in 0.2%, 0.5% and 0.3% [8].
- Population
- Three cross-sectional studies in adults within a review of 50 articles [7]. 3,018 people in three regions of China followed from 1999 to 2004 [8].
- Sources
- [7], [8]
- Caveats
- Observational comparisons between populations defined by their median urinary iodine, not by one person's result. The meta-analysis authors describe the included studies as few and of low quality, and the source of excess iodine was mainly iodized salt or drinking water [7].
Mixed results1
Child IQ and reading when iodine was low in pregnancy
Mixed results: one UK cohort linked low iodine in early pregnancy to lower child verbal IQ, a pooled analysis found a weaker link, and one supplement trial found no effect.
The numbers and sources
- Effect
- In a UK cohort, children of women whose first-trimester urinary iodine to creatinine ratio was below 150 µg/g were more likely to score in the lowest quarter for verbal IQ at age 8 (OR 1.58, 95% CI 1.09 to 2.30) and for reading accuracy at age 9 (OR 1.69, 1.15 to 2.49) than children of women at 150 µg/g or above. The group's median urinary iodine concentration was 91.1 µg/L [4]. A pooled analysis of three European birth cohorts found that a ratio below 150 µg/g was not significantly associated with nonverbal IQ (difference -0.6 points, 95% CI -1.7 to 0.4) or verbal IQ (-0.6 points, -1.3 to 0.1). It found a curved relation with verbal IQ that was present only up to 14 weeks of pregnancy [5]. In a randomized trial in India and Thailand, 200 µg of iodine a day from about 11 weeks of pregnancy did not change children's verbal IQ at age 5 to 6 (89.5 against 90.2 on placebo, difference -0.7, 95% CI -2.9 to 1.5), performance IQ or executive function [6].
- Population
- 1,040 mother and child pairs in the United Kingdom [4]. 6,180 mother and child pairs in the Netherlands, Spain and the United Kingdom [5]. 832 pregnant women with a median urinary iodine of 131 µg/L, and 313 to 315 of their children at follow-up [6].
- Sources
- [4], [5], [6]
- Caveats
- The cohort studies are observational and measured iodine as a ratio to creatinine in µg/g, a different quantity from the concentration on this page. The trial enrolled women with mild deficiency and followed fewer than half of the children to age 5 to 6. All of these studies compared groups of women. None tested acting on one woman's urine result.
Sources
- World Health Organization. Urinary iodine concentrations for determining iodine status in populations. Vitamin and Mineral Nutrition Information System. Geneva: World Health Organization, 2013. WHO/NMH/NHD/EPG/13.1. https://iris.who.int/handle/10665/85972
- Institute of Medicine, Panel on Micronutrients. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press. 2001. https://doi.org/10.17226/10026
- World Health Organization, UNICEF, International Council for the Control of Iodine Deficiency Disorders. Assessment of iodine deficiency disorders and monitoring their elimination: a guide for programme managers, 3rd edition. Geneva: World Health Organization, 2007. ISBN 978-92-4-159582-7. https://iris.who.int/handle/10665/43781
- Bath SC, Steer CD, Golding J, et al. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). The Lancet. 2013. https://doi.org/10.1016/S0140-6736(13)60436-5
- Levie D, Korevaar TIM, Bath SC, et al. Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data. The Journal of Clinical Endocrinology and Metabolism. 2019. https://doi.org/10.1210/jc.2018-02559
- Gowachirapant S, Jaiswal N, Melse-Boonstra A, et al. Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. The Lancet Diabetes and Endocrinology. 2017. https://doi.org/10.1016/S2213-8587(17)30332-7
- Katagiri R, Yuan X, Kobayashi S, Sasaki S. Effect of excess iodine intake on thyroid diseases in different populations: A systematic review and meta-analyses including observational studies. PLOS ONE. 2017. https://doi.org/10.1371/journal.pone.0173722
- Teng W, Shan Z, Teng X, et al. Effect of Iodine Intake on Thyroid Diseases in China. New England Journal of Medicine. 2006. https://doi.org/10.1056/NEJMoa054022
- König F, Andersson M, Hotz K, et al. Ten Repeat Collections for Urinary Iodine from Spot Samples or 24-Hour Samples Are Needed to Reliably Estimate Individual Iodine Status in Women. The Journal of Nutrition. 2011. https://doi.org/10.3945/jn.111.144071
- Andersen S, Karmisholt J, Pedersen KM, Laurberg P. Reliability of studies of iodine intake and recommendations for number of samples in groups and in individuals. British Journal of Nutrition. 2008. https://doi.org/10.1017/S0007114507842292
- Perrine CG, Cogswell ME, Swanson CA, et al. Comparison of Population Iodine Estimates from 24-Hour Urine and Timed-Spot Urine Samples. Thyroid. 2014. https://doi.org/10.1089/thy.2013.0404
- Berger MM, Shenkin A, Schweinlin A, et al. ESPEN micronutrient guideline. Clinical Nutrition. 2022. https://doi.org/10.1016/j.clnu.2022.02.015