Evidence-Based Nutrient Recommendations

Iodine

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by Jack Norris, RD

Contents

Iodine Deficiency and Excess

Iodine is required for a healthy thyroid. For centuries, iodine deficiency has been a problem in many regions due to a lack of iodine in the soil and food supply (Dasgupta, et al.).

Iodine deficiency among those pregnant and breastfeeding can inhibit brain development in the fetus and infant. Both too much and too little iodine can result in hypothyroidism leading to an enlarged thyroid gland, known as a goiter.

In adults, hypothyroidism can lead to mental impairment, apathy, fatigue, weight gain, cold intolerance, constipation, and hair loss (British Thyroid Foundation, 2019; Linus Pauling Institute, 2015; National Institutes of Health, 2020). A systematic review found that subclinical hypothyroidism is associated with an increased risk of depression (RR 2.35, 95% CI 1.84 to 3.02) (Loh, 2019).

Excess iodine can also result in hyperthyroidism, which can cause weight loss, tachycardia (high pulse rate), muscle weakness, skin warmth, and hair loss (British Thyroid Foundation, 2019; Linus Pauling Institute, 2015). There may be a connection between excess iodine and acne (Arbesman, 2005; Danby, 2007).

Iodine Antagonists

Soy, flax seeds, and raw cruciferous vegetables (broccoli, Brussels sprouts, cauliflower, and cabbage) contain goitrogens that counteract iodine (NIH, 2020). In large amounts, or combined with iodine deficiency, goitrogens can exacerbate iodine deficiency or cause a goiter.

Theoretically, consuming large amounts of goitrogens could have a negative impact even when iodine intake is adequate; however, we haven’t found evidence of this except in people who are exposed to unusually high amounts of goitrogens, such as those whose diets are largely based on cassava root (Chandra Amar, 2015).

Excess Iodine and Disease

Excess iodine can cause either hypothyroidism or hyperthyroidism. Often, the condition is due to iodine toxicity from medications or medical procedures, and is temporary; though in some cases it persists (Leung, 2014). Farebrother, et al. (2019) point out:

  • Most adults without underlying thyroid disease and living in iodine-sufficient areas can tolerate a chronic excess iodine intake of up to 2 g/day without clinical effect.
  • High iodine intakes are a particular problem in populations with inadequate iodine that begin a supplementation program; intakes as low as 300 µg/day can trigger hyperthyroidism in individuals who are susceptible.
  • The prevention of iodine deficiency through iodine fortification generally outweighs the risks of iodine excess, but monitoring should be conducted.

Dasgupta, et al. report that Japan has a high iodine intake ranging between 700 and 3,200 μg/d with excellent thyroid health, suggesting that excess iodine might be harmful only in areas where the population has adapted to low iodine intakes.

Iodine and Mortality

A U.S. study estimated mortality rates according to urinary iodine concentration (UIC), utilizing a nationally representative sample of 12,264 adults, aged 20–80 years, enrolled in the National Health and Nutrition Examination Survey (NHANES) III from 1988 to 1994 (Inoue, 2018). The participants were followed for 19.2 years, and those with very high iodine levels had a higher rate of mortality while those with a very low UIC did not.

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Iodine Sources

Iodine is consistently found in only a few foods: iodized salt, seaweeds, sea animals, and dairy products. Cows are given iodine supplements, and iodine in cleaning solutions are transferred to the milk.

In plant foods, iodine is found inconsistently depending on the iodine content of the soil; food grown near the ocean tends to be higher in iodine. Unfortified commercial plant milks are very low in iodine (Ma, 2016; Bath, 2017). Plant milks fortified with iodine have an iodine content similar to cow’s milk but few are iodine-fortified (Bath, 2017; Vance, 2017).

If someone regularly eats seaweed (multiple times a week), they’ll likely obtain adequate iodine; however, the availability of iodine from seaweed is variable and it can provide excessive amounts.

Kelp Supplements

Most iodine supplements are simply tablets made from kelp. Being a seaweed, kelp likely contains at least small amounts of arsenic. There are some very rare cases in which people taking kelp supplements have developed symptoms of arsenic toxicity (Amster, 2007).

A survey of kelp supplements in the U.S. found that 8 out of 9 batches contained some level of arsenic (Amster, 2007), while a survey in the U.K. of imported seaweed found very little arsenic in kelp (Rose, 2007). In contrast, a 2017 ConsumerLabs.com report found arsenic contamination in only 1 of 6 supplements in the U.S., but they found excessive iodine in many of the supplements.

In their 2019 review, Farebrother et al. say, “A survey of the U.S. prenatal multivitamins found significant discordance between label information and laboratory assay; 25 brands containing kelp contained between 33 and 610 µg per daily dose, the latter being almost three times the 220 µg recommended daily intake. This may be explained by the natural variations in kelp iodine content, and for this reason, kelp supplements should generally be avoided, and potassium iodide should be used in vitamin preparations.”

Iodine Status of Vegans

Research has raised concerns about the iodine status of vegetarians and, especially, vegans. In this section, we examine that research to determine how much concern is warranted.

Urinary Iodine Concentrations of Vegans

Average urinary iodine concentration (UIC) is used to determine the iodine status of a population. The World Health Organization (WHO) suggests these categories (WHO, 2007):

iodine-WHO-adequacy.png

But there are problems with using the WHO’s cutoffs to determine the iodine adequacy of an adult population. Iodine status has primarily been studied in school-age children, and Zimmermann and Andersson (2012) point out that the WHO extrapolated adequate UIC levels for populations of school-age children to adults; they provide evidence that an adequate average UIC for an adult population is about 60 to 70 µg/l.

There’s also high day-to-day variability in iodine intake and water consumption for any individual, with fasting samples and morning samples giving lower UIC values (Soldin, 2002). In order to compensate for the various issues with determining UIC, it’s recommended that 500 samples (not necessarily people) be used to determine the UIC for a population (Zimmermann, 2012). In contrast, a total of 360 vegans have been included in all studies, with 212 being the highest number of samples in any individual study (Schüpbach, 2017). The UIC sampling methods and the number of people included in studies on vegans are listed in the Summary tab of Iodine Supplementary Material for VeganHealth.

The chart below includes studies measuring the UIC of vegetarians and vegans. The typical UIC of vegans, depicted by the solid green line, is far below the 100 µg/l that the WHO recommends (the dotted lines are explained in the next section, UIC and Urine Volume).

iodine-diet-vs-UIC-2.png

Table Notes: LOV – lacto-ovo-vegetarian. Sources (from left to right): Finland: Elorinne, 2016; U.K.: Lightowler, 1998; Germany: Weikert, 2020; Norway: Henjum, 2018; Norway: Groufh-Jacobsen, 2020; Norway: Brantsaeter, 2018; Switzerland: Schüpbach, 2017; Slovakia: Krajcovicová-Kudlácková, 2003; Boston: Leung, 2011. (Spreadsheet)

UIC and Urine Volume

None of the studies measuring the UIC of vegans adjusted for urine volume, which could be higher for vegans due to plant foods containing more water, resulting in artificially low UIC measurements. One study has measured urine volume of people while on a lacto-ovo-vegetarian diet (Siener, 2002). In the table below, their data was used to create a factor to adjust the UIC of lacto-ovo-vegetarians and vegans for urine volume. The dotted lines in the Diet Type vs. UIC figure (above) represent what the UIC for lacto-ovo-vegetarians and vegans might be if they were adjusted for urine volume, though this should be considered purely hypothetical.

iodine-urine-volume-7.png

Table notes: 10 men were placed on four different diets to determine the impact of diet modifications on urine composition and the risk of calcium oxalate crystallization. The men consumed a self-selected diet for 14 days, and then three different diets for a period of 5 days each: a Western-type diet “representative of usual dietary habits,” a normal mixed diet, and a lacto-ovo-vegetarian diet. For the normal mixed diet, urine output was 1.5 l/day, which is often cited as the daily urine output for adults (Zimmermann, 2012). During the lacto-ovo-vegetarian phase, urine output was 2.5 l/day due to the higher water content of fruits and vegetables. To adjust for the likely higher water content of a vegan diet, the factor for vegans is based on the same difference in urine output between vegans and lacto-ovo-vegetarians as was found between lacto-ovo-vegetarians and the Western-type diet.

UIC and Creatinine

Soldin (2002) reports that urinary iodine to creatinine ratio (UI/Cr) is considered a more reliable measurement of iodine excretion than UIC measurements based on urine volume. Only one study of vegans included the UI/Cr ratio (Brantsaeter, 2018). The UIC ratio of meat-eaters and lacto-ovo-vegetarians to vegans is higher when based on urine volume than on creatinine (see table below), which provides more evidence that urine volume could be artificially lowering the UIC of vegans. On the other hand, there’s evidence that vegans have lower creatinine levels than meat-eaters (Lindqvist, 2020) which would counteract these ratios, making it hard to assess the UIC of vegans using creatinine.

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Iodine Supplementation and UIC among Vegans

An interesting phenomenon appears when graphing the reported iodine intakes versus UIC in the studies on vegans that reported both: There’s actually an inverse association between iodine intakes and UIC.

iodine-intake-UIC.png

For reference, the United States dietary recommended allowance (RDA) for iodine is 150 µg/day for adults.

Limiting the studies to vegans whose intakes were measured but who weren’t adding iodine through seaweed or supplements produces a similar result as seen in the table and graph below.

iodine-intake-IUC-non-suppl-table.png

iodine-intake-IUC-non-suppl-graph.png

The study by Lightowler and Davies (1998) is worth a closer look. They used a a chemical analysis of duplicate portions of 4 days of food and urine to determine iodine intake and urinary iodine output of vegans in the U.K. They found that of the 30 vegans, 22 weren’t taking supplements or seaweed, 3 were taking seaweed, and 7 were taking supplements. The chart below shows their intakes and urinary iodine excretion. Iodine intakes for those eating seaweed were 866 µg/day. The results suggest that either the iodine in the seaweed wasn’t bioavailable, the intakes were misreported, or the iodine had cleared by the time the spot test was conducted. Adjusting for higher urine volume (as explained in UIC and Urine Volume) increases the UIC for supplementing vegans to the 60 to 70 µg/l range recommended for an adult population by Zimmermann and Andersson (2012).

iodine-Lightowler-UK-vegans-graph.png

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Thyroid Hormone Levels in Vegans

With regard to the iodine status of individuals, Zimmermann and Andersson (2012) point out that UIC isn’t an appropriate indicator:

A common mistake is to assume that all subjects with a spot UIC <100 mg/L are iodine deficient. But dietary iodine intake and therefore UIC are highly variable from day to day. In iodine-sufficient countries where most iodine intake comes from iodized salt, UIC (both spot and 24-h urine collections) show an individual day-to-day variation of 30–40%.

Due to the limitations of assessing the iodine status of individuals based on UIC, we’ll turn our focus to the thyroid hormones.

Thyroid Stimulating Hormone in Vegans

Less studied in vegans than UIC, but arguably more important, is thyroid-stimulating hormone (TSH). Although TSH is considered a poor indicator of iodine status, it’s a more direct indicator of thyroid health which is the objective of maintaining a healthy iodine status.

Normal TSH levels are typically considered to be between 0.5 to 5.0 mIU/l. In general, a higher TSH level is a sign of hypothyroidism in which more TSH is signaling the thyroid to produce the thyroid hormones T3 and T4. A lower TSH level is a sign of hyperthyroidism in which the body is not signaling the thyroid to produce more hormone because there’s already too much.

There are four studies reporting TSH levels in vegans (Key, 1992; Leung, 2011; Rauma, 1994; Weikert 2020). Except for a caveat regarding Key et al. (explained in the chart notes), the average TSH levels of the total of 150 vegans in these studies were, respectively, 2.2, 1.1, 2.1, and 2.4 mlU/l; all well within the normal range.

Using the best data we have and except when iodine intake is excessive, the chart below shows normal TSH levels for a range of usual iodine intakes among vegans.

iodine-intake-v-TSH-6.png

Chart notes: An iodine intake of 87 µg was estimated for the vegans in Key et al. who weren’t supplementing. This was based on a direct measurement of dietary iodine conducted in another study on U.K. vegans who also weren’t supplementing (Lightowler, 1998). The U.K. has never had a program of iodine fortification (Vanderpump and Bath, 2019; Patience, 2018) and so the vegan intakes should have been similar in the two studies, but regardless of the actual iodine intake of the vegans in Key et al., their average TSH level was normal.

Key et al. excluded three vegans from the average TSH measurement due to reporting taking kelp. These three vegans had very high TSH measurements of 11.3, 13.4, and 26.4 mlU/l and are excluded from the graph above of Iodine Intake vs. TSH. For these participants, there’s a small chance of reverse causality such that some were supplementing with iodine in response to a diagnosis of a high TSH. The remaining vegans in Key et al. reported taking no iodine supplements; two of these had elevated TSH levels of 5.3 and 8.3 mlU/l (and are included in the graph above).

Rauma et al. studied a small group of long-term, vegan raw foodists in Finland, many of whom had adopted the diet to treat medical conditions. Chemical analysis of duplicate food portions revealed an intake of 26 µg per day. All vegans had a TSH of under 4.0 except the vegan with the highest urinary iodine of 1,700 µg/day who had a TSH of 5.0 mlU/l (laboratory reference range was to 0.2–4.0 mlU/l).

One exception to the normal TSH levels among non-seaweed-eating vegans is reported in an obscure abstract (Crane, 1992) that I couldn’t locate, but that is described by Remer et al. The study was of vegans in the United States who used only non-iodized sea salt and had significantly reduced serum iodine concentrations; 25% had clearly elevated TSH levels and 12% had developed hypothyroidism.

Thyroxine (T4) in Vegans

The thyroid produces two hormones that regulate metabolism: triiodothyronine (T3) and thyroxine (T4). The thyroid mainly produces T4 (Farebrother, 2019). The half-life of T4 is 5 to 7 days, while the half-life of T3 is only 1 day (Medscape, 2016).

In the three studies measuring thyroid hormone levels of vegans, only one vegan had a T4 level below the reference range, and that was a participant in Key et al.’s study who was supplementing with seaweed (table below). Note that Leung et al. excluded participants who had thyroid disease, so only undiagnosed thyroid disease would have been detected in these subjects.

iodine-T4-4.png

Thyroid Status in Adventist Health Study-2

The most direct way to determine the thyroid status of vegans is to measure their rates of thyroid disease. The data is limited, but we have reports from the Adventist Health Study-2 measuring the prevalence of hypothyroidism and hyperthyroidism.

The prevalence of hypothyroidism among Adventist Health Study-2 participants from the U.S. and Canada was reported both cross-sectionally and prospectively after about 4-6 years of follow-up (Tonstad, 2013). Lacto-ovo-vegetarians were more likely to have been treated for hypothyroidism in the previous year in the cross-sectional arm, while vegans had a trend towards less hypothyroidism in the prospective arm.

iodine-AHS2-hypo.png

According to the authors, “While vegan diets are associated with lower body weight, which may protect against hypothyroidism, the lower risk among vegans existed even after controlling for [body mass index] and potential demographic confounders.” It’s not clear why lacto-ovo-vegetarians would have fared worse than meat-eaters. Adding salt to foods daily or more often than daily (compared to less than weekly) was associated with increased hypothyroidism among the entire population; this finding couldn’t be readily explained.

The Adventist Health Study-2 released a report measuring the prevalence of hyperthyroidism (Tonstad, 2015). Compared to meat-eaters, vegans had a similar prevalence of reporting having had hyperthyroidism in the previous year, though only half the prevalence of being treated for it.

iodine-AHS2-hyper.png

Conclusion for the Iodine Status of Vegans

Without supplementation or seaweed, vegans generally have a low iodine intake. Studies of vegans also show them to have lower UIC than omnivores and typically well below the threshold of 100 µg/l that the WHO recommends for a population.

There are three potential problems with using UIC as a marker for the iodine status of adult vegans: 1) the WHO UIC threshold levels are likely too high for adult populations, 2) vegans may have a higher urine volume, and 3) there is a much lower number of UIC samples in studies of vegans than what is recommended. Taken together, UIC measurements have likely exaggerated estimates of iodine deficiency among vegans.

Increasing iodine intake among vegans does little to improve their UIC, and aiming for an adequate UIC, as defined by the WHO, might be a futile or even counterproductive strategy.

Direct markers of thyroid health, TSH and T4 levels, or a diagnosis of hypothyroidism or hyperthyroidism, tend to be normal in vegans, except in some cases of supplementation with seaweed or kelp supplements.

Recommendations

The United States RDA for iodine for adults is 150 µg, with an upper limit of 1,100 µg. Although most adults can safely metabolize 1,100 µg, and even more, it’s prudent not to supplement beyond the RDA.

Ideally, we’d only recommend supplementing with enough iodine to make up for any difference between usual vegan intakes and the RDA, but we have little data on iodine intakes in U.S. vegans. In the one study on U.S. vegans, Leung et al. (2011) didn’t assess iodine intake, though 24 of the 62 vegans were taking a multivitamin with iodine, and two were taking either kelp or an iodine supplement. This group had the highest UIC that’s been measured to date among vegans.

When you consider that 1) Zimmermann and Andersson report that in healthy adults the mean daily turnover of iodine is approximately 95 µg, 2) without supplementation, vegan iodine intakes across all countries range from 20 to 90 µg/day, and 3) research has provided little evidence of vegans showing clinical signs of iodine deficiency, it would seem likely that supplementing with an additional 100 µg/day would be plenty for vegan adults. However, without more direct research assessing vegan iodine needs, recommending less than the RDA could be risky, especially for non-adults, or for people considering becoming pregnant or who are pregnant or breastfeeding.

If salt is iodized, it will say so on the package. Although iodized salt can sometimes contain higher or lower amounts of iodine than advertised on the label (Dasgupta, 2008), salt iodization is viewed as one of the safest and most effective methods for achieving iodine sufficiency across a population and is not generally considered to be a source of excess iodine (Leung, 2014). Dasgupta et al. found that the iodine on the surface of salt may be lost to oxidation and that humidity can lead to some iodine loss from salt, so it’s best to keep the container sealed and, if you live in a humid environment, store the salt in the refrigerator.

In the United States, salt is fortified with 100 µg of iodine per one-third teaspoon (which provides 774 mg of sodium). Salt in commercial and processed foods is usually not iodized (Dasgupta, 2008), and sea salt doesn’t contain appreciable amounts of iodine unless fortified.

Vegans who don’t meet their iodine needs by way of table salt should opt for a multivitamin or supplement containing potassium iodide. When purchasing, be aware that many stand-alone potassium iodide supplements are intended to block radiation and are measured in milligrams (mg) rather than micrograms (abbreviated as µg or mcg) and can be many times the upper limit.

We recommend not relying on kelp supplements for iodine, but if someone has been taking a multivitamin with kelp, the risks are probably not high enough to discard them; when possible, opt for potassium iodide instead of kelp.

For the U.S. RDA for various age groups, please see Daily Needs.

Appendix A: Studies Measuring Iodine Intakes via Food Tables

Although measuring iodine intakes via food tables is of questionable value due to high variability, for completeness this table lists the studies since 2015 that aren’t included in the above article.

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Appendix B. Iodine Excretion while on a Lacto-Ovo-Vegetarian Diet

An experimental study in Germany placed 6 healthy adults on four separate diets for 5 days each, separated by a 9 day washout period (Remer, 1999). The diets were: 1) a normal omnivorous diet moderate in protein (ND), 2) a normal omnivorous diet high in protein (P), 3) a lacto-ovo-vegetarian diet (LOV), and 4) the ND diet again. They asked the participants not to use iodized table salt and to avoid iodine-rich foods in order to obtain a representative iodine content.

A chemical analysis revealed low amounts of iodine: 15.6 µg/day for the LOV and 35.2 µg/day for the ND. Urinary iodine output was 36.6 µg/day for the LOV compared to 50.2 µg/day for the ND.

The authors conclude that thyroid iodine losses must have contributed to the urinary iodine output and that a steady metabolic state had not been achieved. At the time, Germany was still experiencing endemic goiter with about 50% of the population having enlarged thyroid glands (Hampel, 1995). Subsequently, Germany initiated an iodized salt program (Khattak, 2016).

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Vanderpump M. Bass S. Historical iodine status in the UK. The UK Iodine Group. Updated April 2019. Accessed January 12, 2021.

Zimmermann MB, Andersson M. Assessment of iodine nutrition in populations: past, present, and future. Nutr Rev. 2012 Oct;70(10):553-70.

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