I hear it over and over again…

From a nurse, “She can’t have providine prep, she has a shellfish allergy.”images

From the patient, “I can’t have the contrast because I’m allergic to iodine and shellfish.”

The list of allergies placed in the chart by a doctor, “Radiocontrast. Iodine. Shellfish.”

It’s not the fault of the patients who are typically repeating the medical misinformation they have been given. However, the medical professionals who perpetuate the iodine-shellfish-radiocontrast allergy drive me batty because they either don’t understand the medicine involved or understand there is no link but don’t take the time to educate the patient and correct the chart.

In short, I hate medical myths. Undoing what someone once said to somebody is hard because when someone has believed something for so long (whether they are a doctor, nurse, or patient) they are not always open to a change. I estimate that it takes a minimum of three conversations to under one incorrect piece of information.

Let’s break it down.

While shellfish allergy is a very real allergy and potentially life threatening, it is not an iodine allergy. The major allergens in shellfish are tropomyosins, which are proteins is the muscle and definitely not iodine. Tropomyosins cross react among various shellfish, but not scaled fish so that is why you can be allergic to crab and eat salmon. People who are allergic to scaled fish are typically allergic to a different muscle protein, parvalbumin.

Iodine is not an allergen. We all have iodine in our bodies. It is in our thyroid hormones and in amino acids. We would die without iodine. Iodine deficiency is such a potential health problem that most table salt in the United States contains iodine. Listing iodine as an allergen in the chart is wrong. It is worth repeating, iodine cannot be an allergen.

People can have reactions to providone-iodine prep (which contains iodine), but this is due to allergens in the solution not the iodine. If someone has a reaction to providone-iodine prep the prep should be listed as the allergen, not the iodine. It’s an important distinction. With many products it’s the additives that drive an allergic reaction.

People can have severe reactions to radiocontrast, but these are not allergic reactions. Anaphylaxis is due to a the immune system producing IgE immunoglobulins in response to an allergen, such as the tropomyosin in shellfish. It is an allergic response. When a person is re-exposed, the allergen-IgE complex triggers the severe inflammatory cascade. Reactions to radiocontrast are believed to be anaphylactoid and so are not caused by IgE. What’s the difference? Anaphylaxis requires IgE to trigger the inflammatory cascade and with an anaphylactoid reaction the substance directly stimulates the inflammatory cascade, no immune system intervention is needed. What causes triggers anaphylactoid reactions with radiocontrast? Not the iodine, but likely the hyperosmolarity because  hyperosmolar solutions are highly irritating. As expected, anaphylactoid reactions are much less frequent with the lower osmolar radiocontrast solutions more commonly used today.

As an aside, reactions to radiocontrast (especially the older, more hyperosmolar solutions) are increased about three-fold in people with allergies to milk, eggs, and chocolate which according to the American Academy of Asthma, Allergy, and Immunology indicates a, “general atopic disposition, rather than an iodine specific reaction.” Basically, people with food allergies have a twitchy inflammatory response system and are at higher risk for non-allergen driven reactions as well.

So let’s get rid of the medical mythology surrounding iodine, shellfish, and radiocontrast. When incorrect allergies are listed in the chart it breeds confusion and both patients and their providers need and deserve accurate health information.

Shellfish allergy isn’t iodine driven.

Iodine isn’t an allergen.

Reactions to radiocontrast  are not an allergic reaction to iodine and are unlikely to be allergic reactions at all, but rather an inflammatory response to irritating hyperosmolar agents.

The science matters.


32 replies on “Shellfish, iodine, and radiocontrast allergies: medical mythbusters”

  1. This makes sense, but I’m confused. I had a nuclear contrast in surgery when I was 19 and they said I might develop an allergy to shellfish as a result. I did, they then said I’m allergic to iodine and shellfish. So I’ve operated on the belief that that’s what I’m allergic to. Am I not also allergic to the iodine contrasts?

  2. Can you please help me explain why I
    became highly allergic to shellfish after having a small reaction To the contrast dye even though I have been eating shellfish my entire life with no problems.

  3. So if I have a allergy to shellfish I can’t take instaflex for my joints or knee pain? Please respond.
    Thank you

  4. This has soothed my concerns so much. I am atopic (does this translate to English? Soy atopica) so I do sometimes react randomly to things. I have a CT scan due on Monday (enlarged lymph nodes in neck that haven’t gone down for months) and I am pretty nervous. Reading this factual, scientific analysis has made me feel so much better. Besides, if I have an inflammatory response I will be in the best place!! Thank you.

  5. Thank you! This is exactly what I was wondering about. Makes sense and now I am not so worried about getting my upcoming ct scan.

  6. Iodine is essential to the body. I know this. I can never find i I need to keep telling doctors about my shellfish allergy. I’m also an atopic person in general. Had massive inflammatory outbreak that lasted for days and I still don’t know the exact cause.

  7. I’m confused… and scared….I have a known anaphylaxis reaction to shellfish. Ct scan with contrast resulted in anaphylaxis, betadine results in hives. Multivitamins my throat itches and swells so I stop taking them. What’s the common denominator here???

  8. Thank you for this article . For the longest time I have been going in circles calling doctors , searching online without comfort. Our daughter has shellfish/fish allergy and have been afraid to give her multivitamins with iodibe in them as well as salt. I also read that there is iodine in dairy and eggs and she eats that fine.

  9. Hi
    just wanted to join this conversation as yesterday i almost died seconds after being given contrast during a ct scan. I had to be resuscitated by the crash team.
    Apparently in the 28 years that the radiologist has been giving these types of scans she has never seen the worst reaction it is extrememly rare.
    I have recently ordered some natural iodine to help keep my thyroid healthy. one drop diluted in a glass of water a day. Does this mean that i can take it as it was not the iodine contained within the contrast that almost killed me.

  10. I understand what you are saying, but I am still allergic to the contrast due to the providone in it. That also explains why I get an allergic reaction to betadine as it also contains providone. Knowing all of this doesn’t clear up the fact that I am allergic to contrast as well as the topical use of betadine. Dr.s were amazed I got a reaction to the alternate contrast, that is used if people are allergic to the regular one.

    I also can not eat shell fish or take glucosamine products since I got an allergic reaction to it too. This must be a tropomyosin allergy. Glucosamine contains shells from fish. I do eat certain kinds of fish but not shell fish due to the reactions.

    Should I have my medical records changed to show it is providone and tropomyosin instead of iodine? It doesn’t really change anything as I am allergic to the contrast.

  11. Confused too. Blood tests show I am allergic to shellfish. I had a very nasty reaction to contrast dye during a CT scan. Hives, throat closed up, lips swollen, muscles in arms & legs went from tingly to dead weight, couldn’t move at all within about a minute. Luckily nurse came in to adjust something & saw my face blowing up. Radiology said it was iodine, & said I wouldn’t react to MRI dyes because their MRI dyes don’t have iodine. Didn’t proceed with MRI dye because I didn’t trust that they knew what they were talking about, & was pretty freaked out after that reaction When I went to my GP, she said that iodine allergy was incorrect, & I was right not to proceed with MRI dye. Can someone tell me what’s what, before these doctors put me in a big, loud MRI machine alone, full of MRI dye and with only a panic button to push to communicate if I am having a reaction, but how do I push the button if the reaction stops my arms from working! Need real answers!

  12. So I am slightly confused now as well. I had a hives reaction to ct contrast which at the time I adamantly didn’t want with an intense fear that came from nowhere. Don’t really eat fish so noticed nothing until I started to have tingling lips eating sandwiches. Had no idea why until I questioned the bakery and they told me they had changed to iodized salt. I am aware that it is in our body but could it be different infesting a elemental version that is added to foods?

  13. Whatever it’s called, I have had reactions to a) hair dye (causing my scalp to split); b) tattoo ink (which resulted in swelling and the fine lines turned into blobs) and c) contrast dye (which caused my throat to swell shut within minutes). I’ve never liked fish, so can’t really say if I have a reaction to shellfish (and at this point, I have no reason/motivation to take chances anyway). But if I am not “allergic” to iodine, is there a way to determine what part of hair dye I am allergic to? I am getting older and would LOVE to dye my hair, but the last time was so bad. I have searched for more information about what kind of hair dye I might be able to use (had an issue with henna as well) so being able to isolate the offender would be really helpful!

    1. @IShouldbeaMinimalist Unfortunately our system does not make it easy to identify the true culprits. I only have adverse reactions to things, not true allergies, so I can try things to try to pin down the culprit. Unfortunately, our labeling isn’t adequate in many cases–in multiple cases I have pretty much narrowed it down to “flavorings”. “Artificial flavorings” are even more likely to be an issue. Obviously it’s not all of them but the labels never list them so I have no way of figuring out.

      Even more of a pain is that in one case there was no doubt that the evil ingredient wasn’t on the label at all but rather was an impurity in one of the ingredients.

  14. Praise you for your post! Thank you so much, I am reading all about me on this page. I have this, and all these years I and everyone else thought I was crazy- I have severe anaphylactoid reaction to injected dyes, just had one today. Tried to convince doctor that I probably would have a problem, he brushed it off said no one in his entire experience ever had a problem, I had a severe reaction, scared the you know what out of him, I was a first. I also cannot eat fish of any kind, cannot use any kind of iodine in prepared cleansing soaps, or in salt- I use uniodized salt- I do not have “true” allergies, not truly allergic to anything, but many things make me have anaphylactoid type reactions or rashes- wasps and iodine are the worst- nuts, oils, chocolate, fermented, etc, soaps, dyes, perfumes, laundry detergents, shampoos, etc. I always called it chemical intolerance, and didn’t try to explain it to anyone. I also often got mad at myself and doctors for not knowing what was going on with me.

  15. I had a mild case of hives following an MRI with dye a few weeks ago. I’ve never been allergic to anything that I know of. Since then, however, have experienced hives a number of times including this evening after having sushi and seafood. Might the MRI dye have triggered a sensitivity, or something along those lines? A week ago I had a bad case around my abdomen that lasted several days. I attributed it to a probiotic I had started taking, and they did subside when I stopped the probiotic. Thanks so much for your help!

    1. No I am not incorrect. I am right.

      Everyone has iodine in their body. You cannot be allergic to it. You have it in you right now as you are reading this.

      However, you can be allergic to the additives in the skin prep or topical iodine. You can also have an irritant reaction to the iodine skin cleaner, which isn’t allergic at all but still uncomfortable. It is well proven that reaction to shellfish is tropomyosin, not iodine. You can be allergic to tropomyosin and it can be very dangerous.

  16. I am really confused now. Many years ago I had a blood test done where they wiped my arm with betadine before inserting the needle and again afterwards. It caused my arm to swell up. About a week later I needed a CT scan with contrast. They asked about an iodine allergy and I mentioned the reaction to the betadine. As a precaution they used the alternate one for people allergic to iodine. Shortly afterwards as I was getting dressed I noticed my eye was itching but thought it was a piece of hair irritating it. I then went to the Dr to get the results and he noticed my whole eye was jelly like and my throat was bothering me. This reaction was very quickly.
    Several years before this I tried eating shrimp but felt really weird so did not finish eating it.

    At this point no one will give me any contrast for tests. I am told they do not want to take a chance giving it to me.

  17. This is just one of the non-allergic “allergies” that creep into patient’s charts when they shouldn’t. In some cases, like an ACE inhibitor causing coughing, it is not an allergy but is at least a side effect that may keep the patient from taking the drug. To me, a worse problem is when a patient is forever prevented from taking a drug that might actually help them because they had a side effect caused by being given too high a dose (e.g. getting low blood pressure from taking too high a dose of an antihyertensive) then being told they have an “allergy” to that drug.

  18. Okay, I am now a confused patient! I have a severe shellfish allergy. I don’t recall any doctor warning anybody not to use betadine on me. Before valve-replacement surgery, my cardiologist told me that due to my shellfish allergy, for a contrast MRI, they were going to use an alternative dye, which I’m pretty sure was gadolinium. I had an anaphylactic reaction to the gadolinium (or whatever it was) and they had to pull me out of the MRI and dose me with antihistamines. I don’t recall if they gave me steroids. (The antihistamines knocked me out in mid-sentence with the doctor.) When it came time for my angiogram, they went back to the iodine-containing dye and loaded me up with steroids and antihistamines beforehand. I didn’t have a reaction.

    Okay, so fast forward a few years, and doctors from the same organization don’t want to do any tests on me with ANY kind of contrast dye. Granted, I have a well-documented history of reacting badly to, well, everything, but still, I did make it through the angiogram. And from what you are saying, maybe I wouldn’t have even needed all the medication beforehand.

    But how do I KNOW? Is there some kind of test to say whether I would react to the regular, iodine-containing contrast dye? And if not, why hasn’t that been invented? I would think it would be a pretty basic thing to want to test for, considering how much doctors like imagining things.

  19. All I know, as a mere patient, is that I cannot have the iodine based scrubs used or I rash up and soon start sneezing and coughing. And if dyes are injected for images, I have to be prepped with antihistamines first, otherwise the coughing is so violent that no images are possible. And I still cough and itch days later if antihistamines are withdrawn. I don’t care what they CALL it. The thing about not calling it one thing or another, as I’ve observed, is that then some idiot who thinks he/she knows a lot announces to me “Oh, no, that doesn’t mean you are allergic” and insists on administering said problem to me. I get told I am a “bad patient” for argument over such decisions. But I’m the one that has to suffer the reaction, so the docs can call it what they like; I know, for instance that I cannot take Biaxin and I don’t care if the doc thinks my initial reaction of “rash” is not sufficiently allergy like. Because on the heels of the rash comes fits of gasping and coughing….and I don’t want to listen to some doc who does NOT know me ask me “Maybe the cough was from something else?”

    1. As mentioned above, reactions to prep are to other ingredients not iodine. For some people it may be an allergic reaction and for others it is an inflammatory response due to irritation.

      Your reactions are well explained in the posts, so it does not negate your experiences at all. An atopic person might be more likely to react to both prep and contrast. Pre medication with antihistamines is a way to reduce the reactions.

      Your point about medical professionals not knowing the difference between the types of reactions and their causes is sadly on point, hence the post.

      1. Thank you for understanding my concerns! Oddly, my first reaction to an iodine-based dye for a brain scan pre-dated any of my shellfish reactions. And yet, when questioned about shellfish allergies and responding in the negative, this led to me being told “You can’t be reacting to the dye….it must be psychosomatic.”

        When I asked why I would have any psychosomatic reaction to something I expected NO issue with at all, the technician turned on his heel and walked off.

        In spite of needing iodine, as we all do; I note I do seem sensitive….I can’t even take multi-vitamins containing it for more than 3 to five days without a generalized rash and increasing cough. I’m grateful I live on a seacoast where my natural exposure is sufficient.

      2. As a long time CT tech, I have administered IV contrast to many patients.Yes, I have seen reactions in a varying degrees of severity.I have seen patient develop urticaria after a contrasted CT but have no reaction at all went a repeat exam was medically indicated. But it astounds me that doctors and nurses and even the history forms that patients have to fill out perpetuate the shellfish myth. I ordered and read a book titled Allergy Relief by Andrew Young. In the second chapter he warns of the danger of having diagnostic imaging performed with contrast if you are allergic to shellfish. This myth refuses to die.

      3. Hi my name is Rick
        years ago I ate lobster for the first time, that was bad after about 1 hour later, in the ER turning Blue, all these years I always thought it was the iodine, never trying again, I use to eat fish all the time now I never touch it (SAD) I also would like to take some GNC pills but they all say they have Shellfish in them Can I take these pills??

        Thank you

  20. It’s said that it takes no time for incorrect information to enter a medical textbook, whether myth or not; but it takes a century to remove it.

  21. It is so true that it takes a long conversation to undo what should’ve never been said. I see this all the time with antibiotic “reactions.” Child had a virus, got an inappropriate antibiotic, then a viral rash and suddenly has a penicillin allergy…sometimes I don’t even have the energy to fight it anymore.

    1. I had a strong, odd reaction to amoxicillin as a young child- I was told to avoid all penicillin-class drugs because of it. Is this also a medical myth?

  22. There is a major pattern of blaming the obvious ingredient with dealing with allergies and similar non-allergy reactions. Even doctors are guilty–I’ve even had a doc substitute pork for ham on a blinded allergy test. I perfectly well knew the culprit was something that was added in the ham-making process, lacking access to the individual ingredients I had no way of figuring out the exact culprit.

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