It is a common misconception that many people are allergic to iodine, but this is impossible: iodine is an essential trace element, so an "iodine allergy" would be incompatible with life. This misconception is particularly damaging, as it can lead to unjustified bans on healthy foods and necessary therapies, as well as unnecessary use of medication. An iodine allergy is in fact an adverse reaction to iodine contrast agents or other ingredients, but is not caused by the iodine content and often not even necessarily an allergy.

Why is it impossible to be allergic to iodine?

Iodine is an essential trace mineral that is necessary for the proper functioning of the thyroid gland, brain and nervous system development, as well as being involved in immunity and vital for the health of the female reproductive organs and prostate. In our brief summary, we gave a detailed account of its effects and explained why a healthy life is unthinkable without iodine. (1)

Where does this misconception originate?

Certain active substances which contain iodine atoms may indeed cause negative reactions in sensitive individuals. In such cases, however, it is not the iodine that is to blame, but the various specific effects of these complex molecules. (2,3,4,5) Just as in the case of gluten sensitivity (celiac disease), the oxygen atom is not responsible for the allergenic effect (although it is found in gluten protein), so too is the case with iodine.

(6,8) The reason for this misunderstanding is probably that iodine is also used in the name of iodine-containing contrast agents and disinfectants, so that in the case of negative reactions, it is often blamed.
In the case of autonomic hormone-producing tissue in the thyroid gland (e.g. hot flushes caused by a swollen thyroid), inorganic iodine itself can cause adverse reactions (hyperthyroidism), yet this is not an allergy either, but an organ-specific issue, which fortunately is quite rare.

How frequent are negative reactions referred to as „iodine allergies”?

More than 100 million procedures are performed with iodine contrast agents every year, so it is important to be aware of the risks. (4)
A summary published in 2010 detailed the risk of adverse reactions to iodine-containing contrast agents. (3) It is important to stress that the following are not necessarily allergic reactions.

• The incidence of adverse reactions depended mainly on the type of contrast agent used, the severity of the reaction and the history of similar adverse reactions, and ranged on average from 0.2-17%. Fortunately, the majority of these were not severe (e.g. increased sweating), but lasting damage did occur as well.
• Among those who had experienced a previous adverse reaction to contrast agents, the risk of mild reactions increased to 7-17%, but the rate of severe symptoms did not increase.
• Really severe reactions, e.g. acute kidney damage, occurred in only 0.02-0.5%, while death occurred in only 0.0006-0.006%; and none of these were associated with "iodine allergy", seafood allergy or previous contrast reactions.  
• Also, in the case of povidone-iodine used in the popular disinfectant, it is not the iodine that is the problem, but the carrier material, which is an organic polymer.

How about seafood?

An unpleasant part of the misconception is that patients are often banned from seafood because of its iodine content or perceived allergenic effects. Conversely, it is precisely those patients who are allergic to seafood who are afraid of anything with iodine in it. (3,4,5,7)

The main allergens found in shellfish are called tropomyosins, which also have nothing to do with iodine; tropomyosins are cross-reactive allergens with crustaceans and molluscs. The risk of severe allergic reactions to contrast agents may be slightly higher among people who are allergic to certain foods, but this is broadly the same for all major allergens. (2)

Unnecessary medication

It is also a pointless and sometimes harmful practice that steroids and other drugs are often used before contrast agents to avoid a suspected iodine allergy.(3)
These drugs are also not without risk, as they can themselves cause allergic reactions or other negative side effects, and there is no evidence that their use is effective; studies have shown no or minimal reduction in adverse reactions to contrast agents. (3)
A 2015 study reported that the spread of this misconception and the often harmful overmedication could be minimized with proper information. (6)

Summary

There is no such thing as an iodine allergy. Hopefully, in the future, this term will be used less and less, as it is completely unfounded and just scares people away from this otherwise important trace element.
It can also be used to wrongly ban people from necessary therapies, healthy foods and potentially harmful drugs to redundantly treat their perceived allergies.