Iodine and Seafood Allergy: Unconventional Wisdom

I recently evaluated a patient referred for seafood and iodine allergy. As a child she had an episode of vomiting and rash after eating shrimp although the details of the episode were vague since she was only three at the time.  She does recall being told that she was allergic to shellfish and should avoid all fish and shellfish, a recommendation she has followed for the past forty years.

When in her twenties, she had a head injury as a result of a skiing accident (she ran into a tree) and a CT of the head was ordered.   An injection of iodine-based contrast dye was  to be given before the x –ray so that blood vessels and possible bleeding could be seen more clearly. She mentioned her shellfish allergy to the technicians performing the test and so Benadryl and a steroid were ordered by the physician to prevent an allergic reaction to the iodine since it was well known that seafood contained a lot of iodine.

Worrying about an allergic reaction, she became quite anxious during the x-ray and developed a few hives, something that happened fairly often when she became nervous and upset. Since if was not clear what cased the hives, she was given more Benadryl and steroids and it was suggested that she warn her doctors that she was allergic to iodine based contrast materials as well as seafood.

About a month before coming into our allergy clinic, she had been in to see her family doctor for her yearly physical and a small nodule was noticed in her neck. She was sent for thyroid tests and a thyroid ultrasound, which confirmed a thyroid nodule, and so another scan using radioactive iodine was ordered.   She told her doctors that she could not have anything with iodine since she was very allergic to both iodine and seafood. This led to a prompt referral to an allergist.

As it turned out, allergy testing to fish and shellfish (crab, lobster, and shrimp) was  negative.   A food challenge to shellfish was offered and she was able to eat a small amount of crab cake under observation in the office without any problems.

This case is not unusual and highlights a common misunderstanding about seafood allergy and iodine.

Iodine Allergy is a Myth

Iodine allergy does not exist.   Iodine is an element that occurs naturally throughout nature and can be found in the periodic table along with other elements such as oxygen and iron. It is found to a varying degree in most of the food we eat every day. If we had allergies to these common elements we would be in serious trouble.

Shellfish allergy occurs because of sensitivity to a muscle protein called tropomyscin which is found in all shellfish as well as in many insects. If you are allergic to one shellfish, such as shrimp, you are also at risk of reacting to other shellfish such as lobster, crab, and mollusks.

Allergy to fish is caused by sensitivity to a different protein unique to fish and not found in shellfish.   You can be allergic to shellfish and eat fish without difficulty and vice versa.   However, cross contamination, such as might occur in a seafood restaurant where shrimp is cooked in the same pan as the fish, can cause problems.

Not So Wise Conventional Wisdom

So where did the belief that being allergic to seafood put you at risk of being allergic to iodine come from? As it turns out, this is a very good example of a conventional wisdom. Conventional wisdom is a commonly held belief that is assumed to be true by laymen and professionals alike because it has always been assumed to be true and sounds like it should be true. Conventional wisdom can go unchallenged for many years, finding it’s way into text books and  being taught in university, before finally being put to rest by the truth.  Shellfish does contain more iodine than many other foods although this has nothing to do with shellfish allergy.

Both allergic and non-allergic reactions do occur to contrast materials containing iodine although these reactions are not caused by an allergy to iodine and are not related to seafood allergy.   People who have other allergies, such as to pollen, animal dander, or foods, may be at an increased risk of having an adverse reaction to an iodinated radio contrast material, but being allergic to seafood does not place you at any greater risk compared with other allergies.