Iodine
An essential trace mineral, bought with the thyroid and pregnancy in mind.
- Thyroid
- Pregnancy
- Children's development
- Diets low in iodine
- Breastfeeding
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is iodine?
Iodine is a trace mineral the body cannot make and has to take in from food. Its work is done in the thyroid gland, which cannot build its hormones without it. Iodised salt, seafood and grains grown in iodine-rich soil are the main dietary sources, and few countries make salt iodisation compulsory: in Europe, the countries with the most shortfall are the ones without a programme. It is sold as potassium iodide or as dried kelp; the iodine in skin antiseptics and mouthwashes is a different product.
Why do people take iodine?
Thyroid
The thinking. The thyroid is what most people have in mind when they reach for iodine. The gland pulls iodine from the blood and builds it into thyroxine, and it cannot make enough of that hormone when iodine runs short.
What research has found. A review of women in the months after giving birth found thyroid hormones no different with a supplement or without. In a four-week trial in adults who already had enough, doses running up to 2,000 micrograms a day pushed thyroid-stimulating hormone up in every supplemented group, and the higher ones shifted a share of people into a mildly underactive pattern on the blood tests. Research here is still young: a few studies so far, mostly pointing the same way.
Pregnancy
The thinking. Iodine is the supplement most often raised with women who are expecting: the need rises by at least half in pregnancy, and the mother’s thyroid is covering the baby as well as herself. Women who eat no dairy, no seafood or no iodised salt are the group reviewers flag most often.
What research has found. Trials in women who were mildly or moderately short of iodine measured thyroid hormones, thyroglobulin - a blood marker that climbs when iodine is scarce - and the size of the gland. Thyroglobulin came down fairly consistently, pooled results found thyroid-stimulating hormone lower, and the gland grew less over the pregnancy. Hormones, antibodies and the newborns’ readings were mostly no different. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
Children's development
The thinking. For many women the reason is not their own thyroid but the baby’s brain, which relies on the mother’s thyroid hormone while it is forming. Severe shortage during development has long been known to do lasting harm; whether a mild shortfall does is the open question.
What research has found. Randomised trials gave women who were mildly short a supplement from early pregnancy and tested the children years later. The largest, in 832 women, found verbal and performance IQ no different from placebo, and pooled analyses found no difference in cognitive, language or motor scores. Reviews reaching back to regions where shortage was severe, and a review of eight trials in children given iodine directly, did find higher cognitive scores. The studies disagree, so this is unsettled: dose, duration and who took part all seem to shift the result.
Diets low in iodine
The thinking. Iodised salt and seafood are where most iodine comes from, so people who have given up fish or dairy, or who eat little salt, come looking for it. Reviews keep finding low-level shortfalls across Western Europe, most often in girls and women; kelp is the other route people take.
What research has found. A trial in 112 adults who were mildly short gave 150 micrograms a day for 24 weeks and tracked thyroglobulin, which falls as iodine supply comes up: it dropped by 12, 20 and 27 per cent against placebo over that time. A review of brown seaweed found its iodine generally well taken up, but how much a product holds varies enormously. Few studies have measured this yet, so there is little to go on either way.
Breastfeeding
The thinking. Babies are born with very little iodine of their own and take what they need from breast milk, and a mother’s requirement stays high while she is feeding. A tub bought for pregnancy often stays on the shelf afterwards.
What research has found. A review across the first year of feeding found iodine in breast milk highest in the first days and easing down after that, and that daily or large one-off doses raised it. A trial in 241 mother-and-baby pairs compared a single large dose to the mother with one to the baby; the milk route kept the babies supplied for longer. Only a few studies have measured this so far, though they mostly agree.
What does iodine do in the body?
Swallowed iodine is absorbed almost completely by the stomach and upper intestine as iodide, the element in its simplest charged form. The thyroid draws it out of the blood and builds it into thyroxine, the hormone behind metabolism, growth and brain development. Going short therefore shows up across those systems rather than in one organ.
How much is needed is not fixed. It is higher in infancy and childhood, rises by at least half in pregnancy, when the mother’s thyroid is covering the baby as well as herself, and stays raised while breastfeeding, because the breast concentrates iodide and passes it into the milk. The relationship is not one-way either: too much iodine can alter thyroid function as well as too little, in people who are susceptible, and safe upper limits in pregnancy have never been clearly settled.
What do people say about taking iodine?
These are anecdotes people have shared, not evidence.
What people like
- The thyroid is the commonest reason given, usually after a borderline blood test or a practitioner’s suggestion.
- On vegan and vegetarian boards it is the nutrient people most worry about missing once dairy and fish go.
- A small number describe better hair, nails and skin on seaweed capsules, alongside a long list of other supplements.
- Women who ate seaweed in pregnancy before hearing any warning almost all report healthy children, and reassure each other.
Common complaints
- Feeling unwell within a week or two, and a thyroid blood test moving the wrong way afterwards, is described most vividly.
- Not knowing how much iodine is actually in a seaweed tablet is the most repeated worry, on nearly every thread.
- People with an autoimmune thyroid condition report being told, by a doctor or other posters, to keep away from extra iodine.
- Throat discomfort, soreness in existing thyroid lumps and a sense of having stirred something up are described by a handful.
How people take it
- Around 150 micrograms a day is the figure quoted for adults, rising to roughly 200 in pregnancy and breastfeeding.
- Many take it inside a pregnancy or vegan multivitamin rather than separately, checking the label to see whether it is there.
- Others use drops a few times a week, or crush seaweed tablets into stews and curries rather than swallowing them.
- Having thyroid blood tests done first is the step most often urged, particularly where an autoimmune condition is in the picture.
Read from these public discussions:
- How do you get enough iodine? (Mumsnet, 2019)
- Anyone else a vegan ttc? (Mumsnet, 2017)
- Did you eat seaweed salad during pregnancy? Was your baby okay? (Mumsnet, 2023)
- Hypothyroid, talk to me about iodine deficiency, kelp and medication please (Mumsnet, 2012)
- Iodine good or bad? (Patient community forum, 2019)
- Hypothyroidism and iodine (Patient community forum, 2016)
What forms of iodine are there?
| Form | What’s different | Elemental iodine by weight | Per serving, on products we list |
|---|---|---|---|
| Potassium iodide | Iodine as the iodide ion, paired with potassium; the gut takes it up almost completely. | about 76.4 % | 150 µg |
| Kelp | Dried brown seaweed, naturally rich in iodine, with contents that vary widely between products. | stated on the label | 293 µg |
What else is worth knowing about iodine?
- Labels give the elemental iodine per serving, usually in micrograms, and a serving may be more than one capsule. The pregnancy trials used 150 to 300 micrograms a day as potassium iodide.
- Too much matters as much as too little. In a four-week trial in adults who already had enough, a mildly underactive pattern appeared on the blood tests in five per cent of those given an extra 400 micrograms a day, and in 15 to 47 per cent of those given more; the authors cautioned against a total above 800 micrograms a day. Reviewers add that excess in pregnancy is common and can be detrimental to the developing baby.
- The amount of iodine in kelp is not fixed. A review of brown seaweed describes contents that can be very high, are not regulated in the European Union, and are only partly lowered by processing. One trial used kelp with its iodine deliberately reduced, about a milligram a day for eight weeks, and found thyroid hormone levels did not rise.
- The largest review of supplementation around pregnancy found digestive upset reported far more often on iodine than without, from one small trial rated very low quality, and no clear differences in the other outcomes. Almost all of that evidence came from areas of mild or moderate shortfall.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (26 studies)
- Do Postpartum Maternal Iodine Status or Supplementation Affect Thyroid Function After Delivery? A Systematic Review and Meta-Analysis. (2024, meta-analysis)
- Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double-blind trial. (2012, randomised trial)
- Effects of different iodine supplementation strategies on thyroid function in iodine-deficient pregnant women: a meta-analysis. (2026, meta-analysis)
- An update: maternal iodine supplementation, thyroid function tests, and child neurodevelopmental outcomes. (2023, meta-analysis)
- Iodine supplementation for women during the preconception, pregnancy and postpartum period. (2017, meta-analysis)
- A randomized, double-blind study of iodine supplementation during pregnancy in Sweden: pilot evaluation of maternal iodine status and thyroid function. (2021, randomised trial)
- The Effects of Iodine Supplementation in Pregnancy on Iodine Status, Thyroglobulin Levels and Thyroid Function Parameters: Results from a Randomized Controlled Clinical Trial in a Mild-to-Moderate Iodine Deficiency Area. (2019, randomised trial)
- Infant neurocognitive development is independent of the use of iodised salt or iodine supplements given during pregnancy. (2013, randomised trial)
- Iodine supplementation for preventing iodine deficiency disorders in children and adolescents. (2026, meta-analysis)
- Effects of iodine supplementation during pregnancy on pregnant women and their offspring: a systematic review and meta-analysis of trials over the past 3 decades. (2021, meta-analysis)
- Systematic review and meta-analysis of the effects of iodine supplementation on thyroid function and child neurodevelopment in mildly-to-moderately iodine-deficient pregnant women. (2020, meta-analysis)
- Therapy of endocrine disease: Impact of iodine supplementation in mild-to-moderate iodine deficiency: systematic review and meta-analysis. (2014, meta-analysis)
- Iodine and mental development of children 5 years old and under: a systematic review and meta-analysis. (2013, meta-analysis)
- Effect of iodine supplementation in pregnancy on child development and other clinical outcomes: a systematic review of randomized controlled trials. (2013, systematic review)
- Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. (2017, randomised trial)
- The effect of iodine supplementation in pregnancy on early childhood neurodevelopment and clinical outcomes: results of an aborted randomised placebo-controlled trial. (2015, randomised trial)
- Iodine from brown algae in human nutrition, with an emphasis on bioaccessibility, bioavailability, chemistry, and effects of processing: A systematic review. (2022, meta-analysis)
- Iodine Supplementation of Mildly Iodine-Deficient Adults Lowers Thyroglobulin: A Randomized Controlled Trial. (2016, randomised trial)
- Iodine in Human Milk: A Systematic Review. (2018, systematic review)
- Direct iodine supplementation of infants versus supplementation of their breastfeeding mothers: a double-blind, randomised, placebo-controlled trial. (2014, randomised trial)
- Iodine and Iodine Deficiency: A Comprehensive Review of a Re-Emerging Issue. (2022, review)
- Consequences of iodine deficiency and excess in pregnant women: an overview of current knowns and unknowns. (2016, review)
- Infectious Disease Management and Control with Povidone Iodine. (2019, review)
- The effects of iodine deficiency in pregnancy and infancy. (2012, review)
- Porous sorbents for the capture of radioactive iodine compounds: a review. (2018, review)
- Iodine Deficiency and Iodine Prophylaxis: An Overview and Update. (2023, review)
How much does iodine cost?
That depends on the form and the pack, so we work out the price of the same dose of every iodine product we can find and rank them.