Q: Is there a connection between shellfish allergies and iodine?

Q: Is there a connection between shellfish allergies and iodine?

Equating an allergy to shellfish with an allergy to iodine is a fairly common misconception.  Fish and radiographic contrast allergies are also erroneously equated with iodine allergies.

Iodine is essential for proper thyroid function.  Without it, people become ill with thyroid problems.  As it turns out, seafood and crops fertilized with seaweed are a good source of iodine.  As people moved from coastal areas inland, the incidence of thyroid deficiency increased.  Beginning in the 20th century, it became a common additive in many varieties of table salt.  As a result, now you have to work hard to completely avoid it.

The allergen in shellfish is a protein, not iodine.  Some people with iodine allergies really have a topical sensitivity to iodine (e.g., povidone iodine; Betadine), usually a much different kind of reaction than the immediate reaction found with anaphylaxis.

Bottom line: A shellfish allergy should almost never preclude the use of iodine for water disinfection. If you are concerned, get more information about the true nature of the allergy.  If the person has not had problems with other seafood (saltwater) or table salt, iodine is not the culprit.  There are, of course, other reasons for not using iodine as a water disinfectant.

10 comments on “Q: Is there a connection between shellfish allergies and iodine?

  1. Linda Linda

    If this a myth then why when I have had surgery and asked if allergic to anything I respond shellfish and they tag me allergic to iodine?

  2. Admin Admin

    I can’t answer your question as to why the hospital staff would automatically “tag” you as having an iodine allergy based on your history of a shellfish allergy. There is no evidence that an allergic reaction following seafood ingestion is related to the presence or the amount iodine contained.

    It is true that people with seafood allergies have a heightened risk for idiosyncratic reactions to the IV contrast used during certain kinds of x-ray studies. Although these materials contain iodine, based on several studies, most believe that the reactions are not due to their iodine component. By the way, there is also a correlation between a heightened susceptibility to IV contrast and many other foods not connected in anyway to iodine or seafood.

    Bottom line: As I intimated previously, iodine allergies are rare birds indeed. In your case, I am not sure why it would matter to add iodine to your list based on the shellfish allergy. If there is a concern about IV contrast during your surgical procedure, a shellfish (or any food allergy) should be a heads-up that there is an increased risk of a reaction. If you have any more questions, you might find this link helpful. http://www.radiology.ucsf.edu/patient-care/patient-safety/contrast/iodine-allergy


  3. Thorn da Costa Thorn da Costa

    During a stay in hospital with pneumonia I was asked if I had a shellfish allergy before being given an MRI. I queried the question and the nurse made it clear he was specifically concerned because he was using an iodine contrast. Now, he didn’t explain the corallation and everyone was cosciensiouse in avoiding giving a straight answer.
    Do you not think it possible that, while too few to be of statistical relivense, it’s possible there have been instances of some note? Or that it may be a diagnostic tool for sone other conditions that aren’t allergic but medication based concerns.

  4. Alan Alan

    I have a severe allergic reaction to shrimp and lobster. I also noticed after scraping my knee and after methilaide was applied, I had a severe rash break-out on the cut. I was told I was allergic to iodine due to both shellfish and methilaide reactions. I was told shellfish had high concentrations of iodine. I can eat salt, dairy products, potatoes, cranberries, and strawberries (all have good sources of iodine) with no problems. Your thoughts?

  5. Glen Conant Glen Conant

    Just had to add to this conversation….. I have always been allergic to Shrimp and assumed it was the iodine… I was recently in the hospital and given an MRI w/ iodine injection.. Immediately, after the testing, I began to exhibit symptoms of an asthma attack ( shortness of breath).. The Tech. Assis. rushed me back to my room where I grabbed my abuterol and had instant relief… After speaking to the nurse about this later, she stated that, ” If you’re allergic to iodine in shrimp, you’re allergic to iodine. I should not have been given the iodine injection.” I was really surprised because I, too thought it was different…. Remembering back now, the symptoms where different… So, I’m not sure what to think..

  6. Admin Admin

    From what I have read, there is no correlation between the two. An allergy to one does not necessarily mean an allergy to the other. It is possible to be allergic to both, independently.

  7. Dr. Reeves Dr. Reeves

    My practice uses contrast enhanced imaging on a daily basis and it drives ME NUTS when people are labeled as having a contrast allergy if they are sensitive to shellfish. The problem is, the staff continue to perpetuate the myth while my partners continue as well. *Sigh*

  8. Physiology Matters Physiology Matters

    J Emerg Med. 2010 Nov;39(5):701-7. doi: 10.1016/j.jemermed.2009.10.014. Epub 2010 Jan 4.

    The relationship of radiocontrast, iodine, and seafood allergies: a medical myth exposed.

    Schabelman E, Witting M.


    Radiocontrast agents are some of the most commonly used medications in the emergency department. However, both physicians and patients misunderstand the role that allergies play in reactions to radiocontrast media, especially with regards to shellfish and iodine.

    We sought to review the literature describing rates of contrast reactions and risk of contrast administration to patients with iodine allergy, shellfish or seafood allergies, or prior reactions to intravenous iodinated contrast.

    Both authors independently performed literature reviews, including position statements of stakeholder organizations, to gain perspective on important issues. They subsequently performed a systematic search for articles that estimated the risk of administration of iodinated contrast to those with a prior history of contrast reaction, “iodine allergy,” or reaction to seafood or shellfish.

    The risk of reactions to contrast ranges from 0.2-17%, depending on the type of contrast used, the severity of reaction considered, and the prior history of any allergy. The risk of reaction in patients with a seafood allergy is similar to that in patients with other food allergies or asthma. A history of prior reaction to contrast increases the risk of mild reactions to as high as 7-17%, but has not been shown to increase the rate of severe reactions. Severe reactions occur in 0.02-0.5% and deaths in 0.0006-0.006%; neither have been related to “iodine allergy,” seafood allergy, or prior contrast reaction. Low-osmolality contrast media became available in 1988, and many of the higher risk estimates were from the era before it was widely available.

    Iodine is not an allergen. Atopy, in general, confers an increased risk of reaction to contrast administration, but the risk of contrast administration is low, even in patients with a history of “iodine allergy,” seafood allergy, or prior contrast reaction. Allergies to shellfish, in particular, do not increase the risk of reaction to intravenous contrast any more that of other allergies.

  9. Admin Admin

    This is another article that argues against the overlapping connection between shellfish and radiocontrast allergic reactions and iodine as an independent allergen.

  10. David Johnson, MD David Johnson, MD

    Thank you for your comment. Actually, we have addressed this several times over the years – have a look at some of our prior posts.

    As it turns out, there is no correlation between radiocontrast reactions and shellfish reactions.
    It does not mean they can’t happen, you are just no more likely to develop a reaction to shell fish with a history radiocontrast reactions than a person with radiocontrast and a reaction to anything else. In addition, it is not likely that the iodine is the culprit in radiocontrast reactions.

    Having said, I would leave the shellfish alone, too.

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