Iodine deficiency occurs when intake no longer meets the thyroid's needs: 150 µg per day for adults, and 200 µg during pregnancy and breastfeeding. Mild deficiency often goes unnoticed, with a normal TSH level. More severe or prolonged deficiency promotes goitre and hypothyroidism.
The issue is back in the news: in 2024, the WHO Regional Office for Europe warned of an increased risk of deficiency associated with changing eating habits. Here are direct answers to the most frequently asked questions: symptoms, TSH, daily requirements, foods and people at risk.
Key takeaways
- Adults need 150 µg of iodine per day, and 200 µg during pregnancy and breastfeeding.
- Mild deficiency often goes unnoticed, with a normal TSH level; no simple test can diagnose it in an individual.
- Pregnant or breastfeeding women, vegans and people who do not use iodised salt are most at risk.
- Excess intake is also risky: the tolerable upper intake level for adults is 600 µg per day.
What does iodine do in the body?
Iodine is the raw material for thyroid hormones: thyroxine (T4) contains four atoms, and triiodothyronine (T3) contains three. These hormones regulate basal metabolism and, in the fetus and young child, brain development. Iodine contributes to the normal production of thyroid hormones and normal thyroid function, normal energy-yielding metabolism, the normal functioning of the nervous system, the maintenance of normal skin and normal cognitive function (for other ways to support memory, see our dedicated article). The body cannot produce it: it comes entirely from food.
What are the symptoms of iodine deficiency?
Mild deficiency is usually silent. When it persists or becomes more severe, the thyroid enlarges to capture more iodine: this is a goitre, sometimes visible at the base of the neck and sometimes accompanied by nodules. If hormone production eventually falls, signs of hypothyroidism may appear:
- persistent fatigue and slowing down;
- feeling unusually cold;
- unexplained weight gain;
- dry skin and constipation;
- difficulty concentrating.
None of these signs is specific: they have many other possible causes, and only a medical assessment can establish a diagnosis.
Iodine deficiency with normal TSH: is it possible?
Yes; this is actually the most common situation in mild or moderate deficiency. The thyroid compensates: it works harder and may favour secretion of T3, the more active hormone, which contains less iodine. As long as this adaptation is sufficient, TSH and hormone levels remain within the normal range. WHO Europe notes that thyroglobulin is the first marker to respond, and the US NIH states that TSH is not a sensitive indicator of iodine deficiency. The downside of this adaptation is an enlarged thyroid and more frequent nodules over the long term.
How can you tell whether you are getting too little iodine?
There is no simple, reliable test at the individual level. Urinary iodine measurement is the reference tool for assessing a population, but it varies considerably from day to day. A Swiss study found that about ten samples are needed to estimate an individual's status with 20% precision. A single measurement therefore cannot establish a diagnosis. In practice, a doctor relies on a neck examination, ultrasound, TSH and thyroid hormone measurements, and your eating habits.
Is iodine deficiency common in France?
France has no recent nationwide urinary iodine data. According to the INCA 3 study cited by ANSES, average adult intake is around 148 µg per day, just below the reference of 150 µg: part of the population therefore falls below it. Salt iodisation is voluntary in France. In its 2024 report, WHO Europe highlighted two trends affecting intake in Europe: the limited use of iodised salt in processed foods, which provide 70 to 80% of the salt consumed, and the growth of plant-based alternatives to milk, which are rarely fortified with iodine.
Who is most at risk?
- Pregnant and breastfeeding women, whose requirements rise to 200 µg per day. In a British cohort of 1,040 women, children whose mothers had insufficient iodine status in early pregnancy were more likely to fall in the bottom quartile for verbal IQ (odds ratio 1.58) and reading. This was an observational study, and the WHO considers the evidence inconclusive, but the signal warrants attention. Choline is another nutrient to monitor during this period, as we explain in our article on choline.
- Vegans and, to a lesser extent, vegetarians. A review of 15 studies in industrialised countries found that vegans had the lowest urinary iodine levels, and no vegan group reached the optimal range.
- People who do not use iodised salt, or who eat little fish, dairy or eggs.
How much iodine do you need per day?
| Population | ANSES and EFSA reference | WHO recommendation | Tolerable upper intake level (EU) |
|---|---|---|---|
| Adult | 150 µg/day | 150 µg/day | 600 µg/day |
| Pregnancy | 200 µg/day | 250 µg/day | 600 µg/day |
| Breastfeeding | 200 µg/day | 250 µg/day | 600 µg/day |
| Children aged 1 to 10 | 90 µg/day (ANSES) | — | 200 to 300 µg/day depending on age |
The nutrient reference value (NRV) on labels is 150 µg. Iodide, the form found in food and supplements, is more than 90% absorbed.
Which foods are rich in iodine?
| Food | Iodine per 100 g | Example serving |
|---|---|---|
| Raw saithe | 143 µg | 150 g: around 215 µg |
| Baked cod | 130 µg | 150 g: around 195 µg |
| Plain yoghurt | 38 µg | 125 g: around 48 µg |
| Hard-boiled egg | 50 µg | One 50 g egg: around 25 µg |
| Semi-skimmed milk | 26 µg | One 250 ml bowl: around 65 µg |
| Iodised salt | 1,860 µg | 1 g: around 19 µg |
Contents taken from the ANSES Ciqual database; serving values calculated for illustration. Brown seaweeds contain very large amounts, sometimes more than 10,000 µg per dry gram, but levels vary widely: ANSES does not consider them an appropriate way to correct a deficiency.
Should you take an iodine supplement?
A supplement may be considered when the diet is consistently low in iodine (a vegan diet, no iodised salt, little seafood and dairy), or during pregnancy on the advice of a doctor or midwife. In France, iodine supplied by a supplement is capped at 150 µg per day: this is the daily amount provided by Hexagon® Iodine 150 mcg, as potassium iodide. If you have thyroid disease (Hashimoto's, hyperthyroidism or nodules) or take lithium, amiodarone or another iodine-containing medicine, do not take iodine without medical advice.
Is too much iodine dangerous?
Yes. The tolerable upper intake level for adults is 600 µg per day, including during pregnancy and breastfeeding. Above this, excess intake can disrupt the thyroid in either direction: goitre, hypothyroidism or iodine-induced hyperthyroidism. The risk mainly comes from seaweed and supplements containing it: even at the maximum recommended content of 2,000 µg per gram, 500mg of dried seaweed per day is enough to exceed the limit. ANSES advises against these products unless a doctor recommends them in cases of thyroid disease, heart disease, kidney failure, treatment with lithium or an iodine-containing medicine, pregnancy or breastfeeding.
Frequently asked questions
Does sea salt contain iodine?
Very little, unless it is fortified. Non-iodised salt contains around 1.8 µg of iodine per 100 g, compared with 1,860 µg in iodised salt. Fleur de sel, grey salt and pink salt are not iodised unless stated otherwise: look for the word “iodised” on the pack. Replacing your salt with iodised salt, without increasing the amount, remains compatible with reducing salt intake.
Are all fish rich in iodine?
No. White sea fish (cod, haddock and saithe) provide more than 100 µg per 100 g, whereas farmed salmon contains only around 8 µg: over ten times less.
Do plant-based drinks provide iodine?
Rarely, unless they are fortified. In the UK, an unfortified plant-based drink contains around 2% of the iodine in cow's milk, and in 2020 only 28% of non-organic plant-based drinks were fortified with iodine, compared with 88% with calcium. If they replace milk, check the label.
Can fatigue be caused by too little iodine?
It is possible when deficiency causes hypothyroidism, but fatigue has much more common causes: lack of sleep, stress or iron deficiency. Other nutrients also affect energy, such as vitamin B6, which contributes to reducing tiredness and fatigue (see 5 things to know about vitamin B6).
Can iodine deficiency disrupt sleep?
A direct link has not been established. Hypothyroidism may be accompanied by fatigue and drowsiness, but sleep disorders usually have other causes, which we discuss in our article on sleep.
Can you take iodine if you have Hashimoto's disease?
Not without medical advice. In people with autoimmune thyroid disease, intakes considered safe for the general population may cause adverse effects.
Conclusion
Iodine deficiency can be subtle: it may exist with a normal TSH level and cannot be identified by a simple urine test. The best prevention starts with food: iodised salt instead of ordinary salt, white fish, dairy and eggs. Pregnant or breastfeeding women, vegans and people who avoid iodised salt are the first groups concerned; for them, an intake of 150 µg per day may be discussed with a healthcare professional. More is not better: above 600 µg per day, iodine can itself disrupt the thyroid.
Sources
- ANSES (2025). Nutritional references for vitamins and minerals. anses.fr
- EFSA NDA Panel (2014). Scientific Opinion on Dietary Reference Values for iodine. EFSA Journal, 12(5), 3660. doi:10.2903/j.efsa.2014.3660
- EFSA (2025). Overview on Tolerable Upper Intake Levels as derived by the SCF and the EFSA NDA Panel. efsa.europa.eu
- WHO, UNICEF, ICCIDD (2007). Assessment of iodine deficiency disorders and monitoring their elimination, 3rd edition. who.int
- WHO Europe (2024). Prevention and control of iodine deficiency in the WHO European Region: adapting to changes in diet and lifestyle. who.int
- Zimmermann MB, Boelaert K (2015). Iodine deficiency and thyroid disorders. The Lancet Diabetes & Endocrinology, 3(4), 286-295. doi:10.1016/S2213-8587(14)70225-6
- König F et al. (2011). Ten repeat collections for urinary iodine from spot samples or 24-hour samples are needed to reliably estimate individual iodine status in women. Journal of Nutrition, 141(11), 2049-2054. doi:10.3945/jn.111.144071
- Bath SC et al. (2013). 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, 382(9889), 331-337. doi:10.1016/S0140-6736(13)60436-5
- Eveleigh ER et al. (2020). Vegans, Vegetarians, and Omnivores: How Does Dietary Choice Influence Iodine Intake? A Systematic Review. Nutrients, 12(6), 1606. doi:10.3390/nu12061606
- ANSES (2018). Opinion on the risks associated with consuming food supplements and foods containing iodine-rich seaweed (request 2017-SA-0086). anses.fr
- ANSES. Ciqual 2025 food composition database. ciqual.anses.fr
- NIH Office of Dietary Supplements (2024). Iodine – Fact Sheet for Health Professionals. ods.od.nih.gov
- Regulation (EU) No 432/2012 establishing a list of authorised health claims. eur-lex.europa.eu
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
