People with shellfish allergy often avoid iodine products because of a commonly held belief that people with a shellfish allergy also have an allergy to iodine. Having a shellfish allergy, however, does not mean you're also allergic to iodine, according to Cleveland Clinic. It is, however, possible to have an allergy to both shellfish and iodine. Ask your doctor if you should use iodine products if you have an allergy.
Shellfish allergies can occur to any crustacean; having an allergy to one type of shellfish does not mean you'll have an allergy to all of them. You may develop a shellfish allergy even if you've eaten shellfish before without a problem. Allergies to shellfish appear to occur as a reaction to tropomyosin, a muscle protein in shellfish. Shellfish make the list of the eight substances most likely to cause an allergic reaction. While food labels must list shellfish as an ingredient due to its high allergy potential, labels do not need to state whether other foods were processed in the same plant as shellfish.
Iodine reactions may not actually be allergies, which are immune responses to an allergen. Instead, iodine reaction may be idiosyncratic reactions better termed as pseudo allergic or allergy-like reactions. Skin reactions to iodine occur rarely and are usually due to skin irritation rather than an allergic reaction. Systemic effects rarely occur from this type of reaction, the University of California San Francisco states.
According to University of California San Francisco, around 5 percent of people who underwent contrast iodine dye in a large study experienced some type of reaction. The risk of reaction in people with seafood allergy was three times higher than the general population, while people with an allergy to eggs, milk or chocolate had 2.9 times the risk. People with fruit and strawberry allergies had 2.6 times the risk, while people with asthma had 2.2 time increased risk. In other words, having any allergy increases your risk of having an allergic reaction to iodine dye. Around 85 percent of people with shellfish allergy who receive iodine dye will not have a reaction, the UCSF states.
If you've always thought you couldn't take or use iodine because of a shellfish allergy, you may hesitate to take anything containing iodine. Talk with your doctor about the risks and benefits of taking iodine. As few as 2 to 3 percent of people who have reactions to dyes used in contrast studies have an actual allergy to the dye, according to a French study published in the 1999 issue of "Allergy."