Iodine is an essential mineral that is required for the proper functioning of the thyroid gland, brain, and nervous system development, as well as immunity and for maintaining the health of the female reproductive organs (breasts, ovaries, endometrium, cervix) and the prostate. (1,29) During pregnancy, it is particularly important to get adequate iodine, as insufficient iodine during this critical period of life can cause permanent damage to the development of the fetus. Iodine deficiency can cause children's growth to stagnate and their intelligence to decline.(2) 

In adults, severe iodine deficiency can lead to hypothyroidism, thyroid enlargement, but even a milder degree of deficiency can have negative consequences in the organs/organ systems that require iodine. (3) Iodine used to be a popular remedy for infections because of its strong antimicrobial properties. Later on, with the advent of antibiotics, its use declined, but it remains one of the most effective disinfectants to this day. (4) Iodine is found in relatively few natural foods in significant amounts, and the iodine content of these foods is significantly influenced by their place of origin. The introduction of iodized salt has reduced the incidence of iodine deficiency somewhat, but since this only provides minimal doses and not both necessary forms of iodine, it still remains one of the most common deficiency diseases in the world today.(5)  

How much iodine do we need? 

Opinions differ on the adequate daily intake for iodine, but officially 150 µg per day is the recommended intake for adults, while 200-290 µg per day is recommended for pregnant and breastfeeding mothers. (6,7) While these amounts are sufficient to prevent the most severe deficiencies, many people believe that we need much more iodine for the healthy functioning of the whole body.   

The maximum daily intake of iodine that is still considered safe has been set by authorities at 1100 µg, as some research suggests that higher daily intakes of iodine can negatively impact thyroid function and cause symptoms similar to iodine deficiency. (6) This finding has also been criticized as it is based mainly on case studies and research with weak evidence, where other factors may have played a role in the development of symptoms.

Is a higher intake problematic?  

Consuming lots of seaweed is a core part of the Japanese diet, which means that their iodine intake is very high, about ~13 mg per day. (8,30) Depending on the type of seaweed consumed, this exceeds the upper limit of safe consumption, yet in general, the Japanese are much healthier and longer-lived than people in Western countries, in particular, the rate of degenerative diseases affecting the reproductive organs is very low. (9) From an evolutionary perspective, it is also likely that humans have adapted to a higher iodine intake than their current terrestrial diets, because they have lived closer to the seas. Higher than recommended iodine intakes are therefore safe for most people and rarely cause adverse symptoms. The occasional discomforts are typically the manifestations of the discharge of toxic substances (halogens: fluoride, bromine and chlorine derivatives), which the increased iodine intake removes from the body through detoxification, which is ultimately a favorable phenomenon. (There are many effective techniques to speed up this detox process.)

Attention: in the case of autonomous hormone-producing tissues in the thyroid gland (e.g. a hot nodule), iodine should not be taken! Contraindications must be screened out with preliminary tests! 

How to achieve an adequate iodine intake? 

As there can be huge variations in the iodine content of food, it is difficult to determine exactly how much iodine we are obtaining from our diet. (6,10,11) In general, the further away from the sea an area is, the less iodine food deriving from that area contains. The richest sources of iodine are marine algae, which can contain up to several thousand µg per gram. (6) Unfortunately, in many countries, obtaining and consuming algae regularly is difficult, in addition, there may also be concerns about their cleanliness, so it is not all that practical to rely on them alone.  

Although much lower in iodine, other marine fish and seafood are also a good source, but only from the oceans and seas, because the iodine content of freshwater fish is just as variable as that of other foods. (6,10,11)  

There can also be significant variation in the iodine content of dairy products and meat, depending on the amount of iodine consumed by the animals, while for vegetables and fruit the iodine content of the soil is the main determinant. Plant sources contain very minimal amounts and are therefore not considered a good source. (6,10,11) For many people, iodized salt provides most of the iodine in their diet, ~50 µg per gram. (12)  

Iodine is also available in the form of dietary supplements, which usually contain a few hundred µg per serving. Increasingly popular are higher doses of iodine supplementation, in the form of Lugol's solution or other complex supplements containing a high concentration of iodine, which can easily cover the higher iodine requirement which we have evolved to need. However, it is worth reading up on it before starting supplementation, as there are contraindications and it can cause problems if used incorrectly. It is particularly important not to use iodine alone, but to make sure all the important iodine cofactors (e.g. selenium, magnesium, zinc, vitamins C, D and A, iron, etc.), are also provided, because in their absence, iodine supplementation may have unwanted effects. 

Health effects of iodine 

One of the greatest dangers of iodine deficiency is that it can cause permanent damage to the physical and mental development of young children through reduced thyroid function, the most severe form of which is cretinism. (14) Several meta-analyses suggest that even mild iodine deficiency can significantly reduce intelligence levels in children, so supplementation during pregnancy is particularly important. (15, 16)   

Another symptom of severe iodine deficiency is goiter, which is a swelling of the thyroid gland. If iodine deficiency persists for a long time, it produces extreme physical symptoms, which fortunately can be reduced over time by increasing iodine intake. (19) Important: even a slight iodine deficiency can have negative consequences in the organ systems listed above, even if the functioning of the thyroid gland is still fine! 

In one study, participants were given iodine in doses of 100-300 µg per day, which improved their antioxidant status and was shown to have anti-inflammatory effects. (20)  

Doses of 1,500 µg or more may slightly increase the levels of TSH hormone (21), but this is not a pathological phenomenon and does not in itself indicate hypothyroidism. The reason behind this phenomenon is that, after detecting a higher serum iodide level, the body wants to stimulate the function and regeneration of the so-called sodium-iodide symporters (NIS). These are responsible for bringing the iodine circulating in the blood into the cells, thus promoting the uptake of iodine by the follicular cells of the thyroid gland. Most people adapt to the higher iodine intake over time and this effect disappears, and TSH also returns to the normal range. However, this can take up to 6-12 months, depending on the dose of iodine and the optimal supply of cofactors (especially vitamin C). 

Some studies report adverse effects caused by excessive iodine intake, usually after high consumption of marine algae. (22,23) In these cases, it is difficult to know exactly what caused the problem, as the algae besides iodine may also contain other harmful substances such as heavy metals, and other goitrogens, so it is possible that the contaminants , and not the iodine caused the problems. The same is true of iodine contrast agents, which are organic iodine compounds, not inorganic iodine, which would be suitable for iodine supplementation. Many people talk about "iodine allergy" if such contrast agents cause negative symptoms in someone, but these reactions are not caused by iodine, but by the organic compound itself. A review study concludes that iodine allergy is a non-existent term whose use should be discontinued. (24)  

Nuclear disasters can release large amounts of radioactive iodine into the environment, which can cause thyroid cancer. To counteract this, it is recommended to consume high doses (15-130 mg) of potassium iodide, which saturates the thyroid gland with iodine so that radioactive iodine cannot be incorporated. (25) Where this therapy was used after the Chernobyl disaster, there was no significant increase in thyroid cancer, (26) whereas, in countries with an iodine deficiency where it was not used, there was a significant spike in the incidence of thyroid cancer. (27)    

Interesting facts 

Iodine used to be such a popular medicine that the Hungarian-born Nobel Prize-winning physician and biochemist Albert Szent-Györgyi wrote: "When I was a medical student, iodine in the form of potassium iodide was the universal medicine. Nobody knew exactly what it did, but it did something and something very good. We students used to sum up the situation with this little saying: 'If you don't know where, what and why, prescribe ye then KI (potassium iodide).'  

It may seem surprising, but some of the iodine in milk comes from iodine-based disinfectants used in animal husbandry, at least on the farms where they are used. However, this is not to be feared, it has only a small effect on the iodine content of the products and has no negative effects. (28) 

The human body not only needs the iodide form of iodine, but also elemental (molecular) iodine in smaller quantities. These forms perform different functions in our cells, and individual iodine-requiring organs and organ systems prefer one or the other. The thyroid gland and skin contain iodide, the ovaries, prostate and non-lactating breasts contain elemental iodine. We can only be sure if we choose a preparation with which we can provide our body with both forms

Thank you Árvay Bálint for your contributions.

[The cover image of the note shows the mineral form of iodine.]