Iodine Simply Explained

1. What Is Iodine?

Iodine is an essential trace element that the body cannot produce itself. It belongs to the group of halogens. In the thyroid gland, iodine is a building block of thyroid hormones. If iodine is lacking over a longer period, the thyroid gland can become enlarged. Traditionally, iodine is used to prevent iodine deficiency and goitre.

2. Where Is It Found, and What Is It Made From?

Iodine is found in marine algae, marine fish and seafood. Iodised table salt also contains iodine. In soils and drinking water, iodine is distributed very differently depending on the region. For dietary supplements, iodine is obtained industrially from iodine-containing brines and salt deposits, sometimes also from marine algae.

3. What Does Iodine Do? What Research Shows

Children and Growth

In areas with iodine deficiency, iodised table salt was associated with better mental development of children in a study in Ethiopia 910. No difference was found between the groups in growth 10. A controlled study (with a comparison group) with iodine tablets found better results in individual thinking tasks in children with mild iodine deficiency 14. Another study with iodine tablets in pregnant women with mild iodine deficiency saw no difference in the children's mental performance 12. The results apply only to the correction of an existing deficiency, not to additional administration when supply is sufficient.

Goitre (Struma)

In iodine deficiency areas, daily iodine administration in pregnant women slowed the growth of the thyroid gland compared with women without iodine administration 2124. After birth, the thyroid gland became smaller again 1920. A study with a sham preparation found no confirmed difference in thyroid volume 18. In villages in Zaire, the frequency of goitre fell with iodised oil, while it remained the same in villages without iodine 25. A review found a trend towards less goitre with iodised table salt, but this was not clear in all studies 6. In breastfeeding women, the frequency of goitre fell with iodine capsules or iodised salt compared with the start of the study 17.

Iodised Salt and Fortification Programmes

In iodine deficiency areas, iodised table salt improved iodine supply. A large study in Ethiopia with over a thousand pregnant women and their children found less iodine deficiency in mothers and children 10. Another study in Ethiopia advised mothers on the use of iodised salt and found more iodine in the urine of the children than in the comparison group 11. Both studies corrected an existing deficiency. The strength of evidence is low, because the controlled studies were small to medium-sized and largely limited to Ethiopia.

Other Investigations

In iodine-deficient areas, iodised table salt increased urinary iodine excretion 69. With iodine supplementation, thyroglobulin, a protein of the thyroid gland, was lower or increased less than in the comparison groups 1415161821. In infants, urinary iodine excretion remained the same whether the mother received the lower of two tested iodine doses or a placebo 22. Iodine capsules and iodised salt did not differ in the amount of iodine in breast milk and urine in breastfeeding women 23. The studies tested very different forms of iodine in sometimes small groups and often only over months.

4. How Was Iodine Taken in the Studies?

The studies predominantly investigated oral iodine. These included iodine tablets, iodised table salt and iodised oil, sometimes also pure iodine. Some investigations gave iodine via fortified foods or as part of combination preparations with several nutrients. Algae and kelp preparations have a fluctuating iodine content and were not tested as pure iodine.

5. Is Iodine Safe? Side Effects in the Studies

In the studies on iodine supplementation in iodine-deficient areas, predominantly no or only mild, temporary side effects were reported 6181920. In a controlled study with iodine tablets in pregnant women, adverse events occurred similarly often in both groups 12. The studies tested very different forms of iodine, sometimes in small groups and often only over months. Algae, kelp and multi-nutrient preparations are mixtures with fluctuating iodine content; their findings cannot be attributed to pure iodine. It remains open how an additional iodine dose affects people who already have sufficient supply.

All details with tables and sources: full expert page on Iodine