Womens Health

Does Iodine Allergy Mean a Shellfish Allergy Too?

Frederick R. Jelovsek MD, MS

"I am highly allergic to iodine. During surgery recently one of the medical attendants told me if I was allergic to iodine I must be allergic to crab and lobster. I actually years ago had a reaction to crab. Are there any other things I should be wary of?"


Allergies are a serious business. Sometimes a skin rash is the main manifestation of an allergy but it can also range from gastrointestinal upset to asthmatic wheezing and even to death (1). People who are allergic to one compound may also have other allergens they are sensitive to. For some reason certain allergies get linked together even though they may be totally unrelated. This is what has happened with iodine and shellfish. There is no actual relationship between iodine allergy and the various seafood allergies. You could have a reaction to both iodine and to ingested crab, but it is much more likely that you are not truly allergic to either.

A true food allergy such as that to shrimp or crab starts within about thirty minutes after ingestion. The reaction is to a muscle protein in the food. Since a true allergic reaction can be life-threatening, intravenous glucocorticoids, antihistamine and sometimes subcutaneous epinephrine is required just like a bee sting kit for people allergic to bees (2). To know for sure about both the iodine and the crab, you should see an allergist and at least be skin tested or use one of the home tests using IgE antibodies to see.

Most allergies to xray contrast media are to the specific media itself, not necessarily the iodine component. It just so happened that when most of the allergies to xray contrast dyes were initially reported that all of the dyes contained iodine in some form so it was assumed that the iodine was the allergen. There are also allergies to non-iodine containing contrast media which are almost as common as to the iodine containing ones. Therefore you have to have a skin test to the specific contrast media to know for sure.

Does an allergy to iodine mean I cannot have iodized salt?

No, it does not. Iodine is a natural element and part of many proteins in the body. Iodized salt is not known to cause an allergic reaction. Even if you have a skin sensitivity to iodine containing products, you can still ingest iodized salt without worry.

Iodine contact allergies are sometimes seen to providone-iodine (Betadine®) scrub used as surgical preps. In one study, 10 people who had skin reactions to providone-iodine were further tested and only one was found to be skin allergic to the iodine, 5 were allergic to the providone-iodine solution itself and the other 4 just had direct skin irritation reactions but not allergic responses (3).

Iodinated contrast material used in certain x-ray studies has the same variability as far as true allergic responses. One study of 165 people who had reactions to iodine containing radiographic contrast material identified only 2-3% who had true allergic responses as measured by IgE antibody response (4).

Is a shellfish allergy the same as an iodine allergy

An allergy to shrimp or crab or salmon, for example, has nothing to do with allergy to iodine. A person could be allergic to both but the allergy to shrimp is due to a protein in the shrimp, not to iodine.

Identified seafood allergens belong to a group of muscle proteins, such as the parvalbumins in codfish and salmon and tropomyosin in crustaceans (shrimp, lobster and crabs). In addition, tropomyosin is a cross-reactive allergen among crustaceans and mollusks such as oysters and scallops (5). Eating the seafood is not the only way to have an shellfish allergic reaction; it has also been reported to be air-borne in places such as a fish market (6).

If I am allergic to shrimp, am I likely to be allergic to other seafood?

Yes. There are some seafood allergies that tend to run together(7). People allergic to shrimp may also be allergic to crab, lobster, clams and oysters. Salmon, trout and mackerel allergy may frequently occur together just as does cod and tuna, and octopus and squid.

The most accurate way to find out what one is allergic to is through skin testing by an allergist. If someone seems to have multiple seafood allergies, however, the true test is to be given the actual food in a double-blind, placebo-controlled food challenge, administered under the care of an allergist or other physician (8). Blood tests for food antibody specific IgE are not quite as accurate as skin tests or food challenges, however you do not have to be exposed to the allergen (and its probably allergic response) in order to be diagnosed.

Why can I sometimes eat shellfish and other times I get quite sick?

Not all reactions to seafood are allergies to the seafood protein itself. In fact a parasite called anisakiasis which frequently contaminates seafood is a major cause of gastrointestinal upset that many people interpret as a food allergy. This parasite is killed by deep freezing the shellfish so if you find you can tolerate shrimp that has been frozen but sometimes get sick eating fresh shrimp, it may be the parasite contamination rather than a seafood allergy (9, 10).

Also, you may just be having a show of food intolerance and not an allergic response. Food intolerance and food allergies are NOT the same. A food allergy that stimulates the immune system to cause a serious reaction such as:

  • hives, itching or skin eczema
  • swelling of the lips, face, tongue and throat, or other parts of the body
  • wheezing, nasal congestion or trouble breathing
  • dizziness, lightheadedness or fainting
is much more serious than a food that your body may be sensitive to resulting in nausea, vomiting, cramping and diarrhea but not a true allergic response.

What is food intolerance?

Food intolerance is a direct irritation of the body by a food or associated chemical or toxin rather than one which stimulates the body's own immune system to attack itself. Food intolerance may be due to the lack of an enzyme such as lactose which results in many people having a problem eating milk, ice milk or other milk products without becoming bloated along with cramping, diarrhea and excess gas. Sometimes food may contain a toxin from bacteria that causes you to react adversely. This does not mean that you cannot ever eat that food again; it just means you should not eat that food when it is contaminated again. Mushrooms, rhubarb, and spoiled tuna or other fish are foods more likely to be come contaminated with toxins. It can be very difficult to differentiate an intolerance to the food itself or to a substance or ingredient used in the preparation of the food. This is especially true of foods containing lactose, wheat or sulfites. For example, wheat can cause a food intolerance but it also can cause an immune allergic response known a celiac disease or sprue.

On the other hand, if you are truly allergic to a food such as:

  • cow's milk
  • eggs
  • peanuts
  • Wheat
  • soybeans
  • fish
  • shellfish
  • tree nuts
even a small amount could give you a serious health reaction. If you suspect that any food is causing you a serious allergic response, be sure to see an allergist to have it diagnosed thoroughly. Skin tests for allergies are considered the best way to diagnose an immune response but sometimes you may have much less of a reaction by just having a blood test known as an in-vitro allergen-specific IgG or IgE antibody test. The home tests use a drop or two of blood from a finger stick which check IgE allergic sensitivities.

Does an iodine allergy mean I cannot have xray dyes?

Again no, it does not. But you need to let the radiologist know about the iodine allergy. Sometimes the dye used can be switched to a non iodine containing contrast material and at other times it may just be suggested to have a skin sensitivity allergy test to the actual dye to be used if there is not a suitable substitute dye available.

Home Test Kit for Food Allergies
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