An iodine allergy is an allergy to foods or other substances which contain the element iodine. Allergic reactions to iodine can vary for those affected by this allergy, but the most severe reaction is anaphylactic shock, which is fatal if not quickly treated. Less-severe allergic symptoms such as asthma, dizziness, and fever can also occur as a result of an iodine allergy. Most doctors think iodine allergy is extremely rare, and often confused for allergies to other substances.
Doctors must first test for an iodine allergy before administering contrast dye to a patient, due to the possible severe reaction the dye could cause. Apart from the allergy itself, some people are unable to effectively remove the iodine from their bodies. In these cases, the kidneys, which normally cleanse waste and excess fluid from the body, fail to remove the iodine, resulting in elevated levels throughout the body. Tests are necessary, because a known allergy to something like shellfish is not sufficient reason to forgo the use of iodinated contrast dye.
Not all people who have an allergy to some things that contain iodine will be allergic to all things that do. Someone who is allergic to shellfish, which contains iodine, may not be allergic to a topical antiseptic that contains iodine, for example. This seems counterintuitive, but only because an allergy to iodine itself is so rare that it is not common to test for it. Iodine is an essential nutrient, necessary in small amounts for proper thyroid function. What is often seen as an iodine allergy is actually something much different.
When someone has an allergic reaction to shellfish, it is rarely because of the iodine itself, but because of a muscle protein present in these types of sea animals. The presence of iodine in shellfish is almost wholly incidental to the actual allergen. Similarly, when an allergic reaction to contrast dye occurs, this is because of other components of the dye rather than the iodine. Finally, the iodine solution used as an antiseptic contains more than just iodine, and it is the presence of these other ingredients that causes the reaction.
It is also worth noting that those with a so-called iodine allergy are almost never found to be allergic to iodide, a slightly different substance produced naturally by the body. Iodide is an ion that consists of an iodine atom with a single negative charge, indicating one extra electron. This ion forms parts of many compounds, some of which are added in small amounts to table salt to prevent iodine deficiency.