Iodine

What does iodine do in the body?

Iodine is a mineral that the body cannot produce and uses in making thyroid hormones. According to the NIH, thyroid function is regulated by TSH secreted from the pituitary; TSH increases the thyroid’s uptake of iodine and the synthesis of T3 and T4. If iodine is insufficient, TSH stays high and the thyroid gland enlarges; this enlargement is called goiter and reflects the gland’s effort to capture more of the circulating iodine. A healthy, iodine-replete adult has about 15–20 mg of iodine in the body, and 70–80% of it is in the thyroid. ^[1] According to the TEMD guideline, 65% of the weight of T4 and 59% of the weight of T3 is iodine. ^[4]

Thyroid hormones affect metabolic rate and the functioning of many organs. The NIH reports that iodine insufficiency is the most common cause of preventable mental impairment in the world. ^[1] This statement applies to severe, population-level deficiency; a mineral being necessary does not mean that intake above an adequate level will provide extra benefit. In the sections below, the limits of both deficiency and excess are given separately.

How much is the daily iodine need?

According to the US Food and Nutrition Board value (RDA) cited by the NIH, the daily reference in men and women over age 14 is 150 µg; it is 220 µg in pregnancy and 290 µg in lactation. ^[1] In its 2014 opinion, EFSA set the adequate intake (AI) at 150 µg/day for adults and 200 µg/day for pregnant and lactating women; EFSA states that a urinary iodine concentration of 100 µg/L corresponds to an intake of about 150 µg/day in adults. ^[2]

The NIH also reports that WHO, UNICEF and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) recommend a slightly higher intake of 250 µg per day for pregnant women. ^[1] The two values belong to different bodies and different methods; one is an RDA and the other an “adequate intake”.

How did Türkiye reduce iodine deficiency?

Mandatory iodized table salt program

The Ministry of Health reports that the “Program for Prevention of Iodine Deficiency Disorders and Salt Iodization” has been carried out in cooperation with UNICEF since 1994, that all table salt was enriched with iodine with the salt communiqué prepared in 1998, and that adding 25–40 mg/kg of potassium iodate to table salt is mandatory. According to the same text (Türkiye Demographic and Health Survey), the rate of iodized salt use rose from 18.2% in 1995 to 85.4% in 2008. ^[3]

The start date is described differently by source; the differences are given side by side: the Ministry of Health cites the 1998 communiqué; the TEMD guideline writes that legal regulations for mandatory iodization of all table salt were completed in July 1999 and that iodized salt use became widespread from the 2000s; TEMD’s public page says mandatory iodization began in 1998–2000. ^{[4] [5]}

Monitoring data in school-age children

The situation was severe before. In the ultrasound and urinary iodine study by Erdoğan and colleagues in 1997–1999, cited by the TEMD guideline, in 5,948 schoolchildren aged 9–11 in 20 provincial centers, the goiter rate was found to be 31.8% and in no province did median urinary iodine exceed 100 µg/L, regarded as adequate. These provinces were chosen from places where goiter was known to be endemic. ^[4] The later surveys summarized by the guideline are below; sample and scope differ from survey to survey.

In the 2007 status assessment study, the median was 42 µg/L in rural schools and 147 µg/L in urban schools; 73.5% of salt samples were counted as iodized, but 56.5% contained more than 15 ppm of iodine. The TEMD guideline regards increasing iodized salt use in rural areas and monitoring compliance with the codex as the main goal; it also writes that the surveys planned for 2012 and 2015 could not be done because no resources were found. ^[4] The TEMD guideline states that the newest data for school-age children belong to 2007.

The TEMD guideline has two different characterizations within itself: in one recommendation box it describes the country as a “moderately severe iodine deficiency and endemic goiter region” and writes that the problem has been largely solved in city centers and persists in rural areas; in the recommendation box at the end of the section, it states that Türkiye is “still a borderline iodine-sufficient region” and that moderate-to-severe deficiency is thought to persist in some rural regions. ^[4] The public page says that with mandatory iodization and iodized salt use, Türkiye has moved into the “borderline iodine-sufficient” regions and adds that Türkiye’s soils are not rich in iodine. ^[5]

Use of non-iodized salt

TEMD reports that it observes an increase in the frequency of non-iodized salt use among the public without a doctor’s advice; unless a physician advises otherwise, it recommends using refined iodized table salt and not using unrefined rock salt and gourmet salts whose content is unclear or to which additives have been added. ^[5] The NIH also reports that specialty salts such as sea salt, kosher salt, Himalayan salt and fleur de sel are generally not iodized, and that non-iodized sea salt provides 0 µg of iodine per quarter teaspoon. ^[1] This statement by TEMD is an observation; no numerical national measurement is given in the text.

Do iodized salt and salt reduction campaigns conflict?

Because table salt is also the main iodine carrier, there is a tension between salt reduction and iodization. TEMD states that it supports the salt reduction campaigns continuing across the country, that so far these campaigns have not achieved a drop in salt consumption to a level that would create iodine deficiency, and writes that if the campaign becomes effective over time, recurrence of deficiency can be prevented by increasing the amount of iodine in table salt. ^[4] The public page says that patients whose salt is restricted or forbidden because of hypertension, heart failure or kidney failure should talk with their doctors about how to meet their iodine needs. ^[5]

Iodine in pregnancy and lactation

Why does the need increase?

According to the TEMD guideline, in pregnancy iodine clearance increases, thyroid hormone production rises by 20–50% from the first weeks, and iodine accumulates in the fetus and placenta; the thyroid’s absolute iodine uptake in pregnant women increases by 50% compared with normal people. ^[4] According to the NIH, severe iodine deficiency in pregnancy can lead to serious neurodevelopmental disorders, growth retardation, miscarriage and stillbirth in the fetus; intense and chronic deficiency causes cretinism. The NIH also reports that mild-to-moderate maternal iodine deficiency has been associated with an increased risk of attention deficit hyperactivity disorder in children; this association does not show causality. ^[1]

Iodine status of pregnant women in Türkiye

In the national cross-sectional study covering 1,444 women who gave birth in 2018–2019 and their infants (Vural et al., 2021), median urinary iodine in pregnant women was 94 µg/L (25th–75th percentile 52–153). The authors counted <150 µg/L in pregnant women as deficiency: mild deficiency (100–149 µg/L) 21%, moderate (50–99 µg/L) 30%, severe (<50 µg/L) 23%; 74% in total. In newborns the median was 96 µg/L and the deficiency rate 51%. Low socioeconomic and educational level, less access to iodized salt in the household, less exposure to skin antiseptic containing povidone-iodine, a higher number of births and higher deficiency in the mother herself were seen together with a higher deficiency rate in infants; there were also regional differences. The authors concluded that iodine deficiency is still an important public health problem in Türkiye. ^[6]

This result has two limits. First, the NIH and TEMD accept a median of 150–249 µg/L in pregnant women as adequate; however, these thresholds apply to the population-level median, and the TEMD guideline states that a single urine sample is not meaningful for an individual’s status. ^{[1] [4]} Second, the TEMD guideline cites studies with limited numbers of pregnant women in different regions of Türkiye in which median urinary iodine ranged between 77–149.7 µg/L and deficiency was found in 49–90% of pregnant women; from larger studies, a median of 77 µg/L was found in 3,543 pregnant women in Istanbul (deficiency in 90%) and 102 µg/L in 864 pregnant women in Trabzon, and iodized salt use among pregnant women in Trabzon is 90.7%. ^[4] The frequency of deficiency varies by study and region; as in the Trabzon example, even when iodized salt use is high, the median remained below the 150 µg/L counted as adequate.

What do different bodies say about supplements?

Guidelines differ from one another; they are given side by side below:

• TEMD (2020): The pregnancy section of the guideline writes that daily iodine intake in a pregnant or lactating woman should be 250 µg, that, assuming a pregnant woman consuming standard iodized salt in Türkiye has a daily intake of on average 100–150 µg, 100–150 µg/day of iodine support should be given to pregnant women and 200 µg/day of support to a pregnant woman restricting salt. The iodine deficiency section of the same guideline mentions 200–250 µg/day of support for a pregnant woman with salt restriction and, in the recommendation box, recommends giving support of 100–300 µg per day without measuring urinary iodine, according to the permitted iodized salt consumption and the region lived in, with support starting when pregnancy is planned and continuing until breastfeeding ends. ^[4]
• WHO/UNICEF/ICCIDD (as cited by the TEMD guideline): The most effective way for iodine replacement in pregnant women is salt iodization; additional support is needed only if salt iodization fails. According to the TEMD guideline, what matters here is whether the community in which the pregnant woman lives is “iodine sufficient”. ^[4]
• American Thyroid Association and European Thyroid Association (as cited by the TEMD guideline): the ATA recommends additional support in all pregnant women and the 2014 ETA guideline recommends support containing 150 µg per day; the Endocrine Society’s 2007 and 2012 guidelines do not recommend additional support. ^[4]

These are the views of bodies, not personal doses; excess iodine can also impair thyroid function (covered separately below).

Does iodine supplementation in pregnancy increase the child’s cognition?

No, it has not been shown in pregnant women with mild deficiency. In the study by Gowachirapant and colleagues, 832 pregnant women who entered the study at a mean of 10.7 weeks of gestation in Bangalore (India) and Bangkok (Thailand), with a baseline median urinary iodine of 131 µg/L, took 200 µg of iodine per day or placebo until delivery; the main endpoints of the study, conducted in regions where schoolchildren are iodine sufficient, were WPPSI-III verbal and performance IQ at age 5–6 and the BRIEF-P executive function score. For IQ, 313 children were analyzed: verbal IQ difference was −0.7 (95% CI −2.9 to 1.5), performance IQ difference −1.6 (−4.5 to 1.3), executive function difference −0.9 (−6.8 to 5.0); none was significant. The study was funded by the Swiss National Science Foundation, the Nestlé Foundation, Wageningen University and ETH Zurich. ^[8] The NIH reports that two randomized trials of similar design (150 or 200 µg/day, from early pregnancy to delivery) found no effect on the child’s cognitive, language and motor scores at 1.5 or 2 years, and in one no benefit was seen at 5–6 years either. ^[1]

The Cochrane review by Harding and colleagues included 14 trials (more than 2,700 women provided data in 11 trials) and found the data insufficient to reach a meaningful conclusion on the benefit and harm of routine supplementation. Almost all trials were conducted in mild-to-moderate deficiency conditions. With low-certainty evidence, supplementation reduced the likelihood of postpartum hyperthyroidism (RR 0.32; 95% CI 0.11–0.91; 3 trials, 543 women); it increased the likelihood of digestive intolerance in pregnancy (RR 15.33; 2.07–113.70; single trial, 76 women; very low certainty). There was no clear difference in hypothyroidism in pregnancy, preterm birth, low birth weight and neonatal hypothyroidism. The authors state that the findings may not apply to conditions of severe deficiency. ^[7]

These findings do not contradict the knowledge that severe deficiency is harmful: the NIH reports that the effects of severe deficiency on neurological development are well documented and that there are studies suggesting that chronic, moderate-to-severe deficiency lowers IQ by about 12–13.5 points, particularly in children. ^[1] The distinction here is between harm in severe deficiency and the fact that it has not been shown that supplementation improves child outcomes in pregnant women found to be mildly insufficient.

Who gets iodine deficiency, and how is it recognized?

The NIH lists people who do not use iodized salt, pregnant women, vegans and those who consume little dairy, seafood and eggs among risk groups. The TEMD guideline states that the problem in Türkiye persists particularly in rural areas, and the public page also mentions patients on salt restriction. ^{[1] [4] [5]} Symptoms, the limit of the urine test and details in Türkiye are on a separate page.

Detailed guideIodine deficiency symptoms: goiter, hypothyroidism, who is at risk, how is it tested?→

Is excess iodine harmful?

The effect of excess iodine can produce results similar to deficiency. The NIH reports the tolerable upper intake level (UL) as 1,100 µg per day in adults (in pregnancy 900 at ages 14–18, 1,100 µg at ages 19–50); high intake in susceptible people can suppress thyroid hormone synthesis and increase TSH stimulation, causing goiter, high TSH and hypothyroidism, and can also lead to iodine-induced hyperthyroidism, thyroiditis and thyroid papillary cancer; hyperthyroidism can also be seen when iodine is given to treat iodine deficiency. Acute poisoning is rare and generally occurs with gram-level doses. ^[1] In Europe, the EU Scientific Committee on Food (SCF) set the upper limit for adults at 600 µg/day in 2002 and regarded this value as acceptable for pregnant and lactating women too; limits for children range between 200 µg at ages 1–3 and 500 µg at ages 15–17. ^[9] This value is below the US limit cited by the NIH (1,100 µg); the two bodies reached different limits through separate assessments.

The TEMD guideline writes that iodine-induced hypothyroidism is most often seen in people with underlying chronic autoimmune (Hashimoto’s) thyroiditis or previously treated Graves’ disease; that for iodine-induced thyrotoxicosis the risk increases with advanced age, multinodular goiter, latent Graves’ and low baseline TSH, and that impairment of the “escape from the Wolff-Chaikoff effect” mechanism plays a role. In people without risk factors, a significant thyroid disorder generally does not follow contrast agent; even in people with risk factors but normal thyroid function at baseline, the risk of overt thyrotoxicosis is below 1%. ^[4]

How does iodine affect Hashimoto’s or another thyroid disease?

TEMD’s public information page says that taking more iodine than necessary can trigger the development of autoimmune thyroid diseases that can result in an underactive or overactive thyroid; this text does not cite a controlled study. ^[5] The TEMD guideline writes that iodine-induced hypothyroidism arises most often in people with Hashimoto’s thyroiditis or previously treated Graves’ disease, that there should be no haste in treating subclinical hypothyroidism detected in the first months after iodine exposure, and that return to normal function can be seen in a significant proportion of patients; this information is given in the context of large iodine loads such as iodinated contrast agents and amiodarone. ^[4] The decision on iodine supplements and kelp in people with Hashimoto’s, Graves’ or nodular goiter is therefore made with a physician.

Does iodine interact with medicines?

According to the NIH, high-dose iodine can have an additive effect with antithyroid medicines (for example methimazole) and lead to hypothyroidism; potassium iodide together with ACE inhibitors or potassium-sparing diuretics can increase the risk of high blood potassium. ^[1] The TEMD guideline writes that amiodarone, 37% of whose weight is iodine, can cause thyroid dysfunction (in 14–18% of users) and that primary hypothyroidism develops at a rate of about 10% in amiodarone users, particularly those with underlying chronic autoimmune thyroiditis. ^[4] It also states that lithium prevents the release of thyroid hormone from the gland and lowers T4 and raises TSH in 15–40% of patients, most of whom have underlying autoimmune thyroid disease. ^[4] The guideline also states that iodinated radiocontrast agents can both decrease and increase thyroid hormone secretion. ^[4]

Does iodine work in fibrocystic breast disease?

Evidence is insufficient. The NIH cites a double-blind study in 56 women in which, with daily iodine (70–90 µg I₂/kg body weight) for 6 months, pain decreased by 65% and by 33% in the placebo group; and, in 111 women (aged 18–50), with 0, 1,500, 3,000 or 6,000 µg of iodine after 5 months, reductions in pain, tenderness and nodularity in those receiving 3,000 and 6,000 µg were significant compared with placebo and 1,500 µg; no specific mechanism of action has been established and the data are limited. ^[1] These doses are several times the NIH adult upper limit of 1,100 µg; they therefore cannot be compared with amounts found in the daily diet or table salt.

Which foods contain it?

The NIH reports that seaweed, fish and other seafood, and eggs are good sources of iodine; and that iodized salt, milk and dairy products also provide iodine. ^[1] The food table, the risk carried by seaweed, storage of iodized salt and cooking losses are on a separate page.

Detailed guideWhich foods contain iodine? Serving table, iodized salt and seaweed→

How is selenium related to the thyroid and iodine?

Selenium becomes part of the structure of selenoproteins that take part in thyroid hormone metabolism; the NIH’s selenium fact sheet reports that selenium deficiency can worsen iodine deficiency. Meta-analysis results on selenium supplementation in Hashimoto’s (antibody decrease, uncertainty of clinical benefit) and selenium’s narrow upper limit are covered in the selenium guide.

Detailed guideWhat is selenium? Thyroid and selenium evidence, daily need and upper limit→

Evidence table: claim, population and limits

This guide does not recommend a daily or weekly personal dose. For pregnant women, lactating women, people with thyroid disease, people using iodine-containing medicines and patients restricting salt, the decision on iodine is made with the treating physician.