Does Shellfish Allergy Mean Iodine Allergy? The Answer May Surprise You

Health Professional, Medical Reviewer

Do you avoid shellfish because of iodine allergy?Have you avoided foods or certain medications that contain iodine because of shellfish allergy?Did you know many types of seafood, in addition to shellfish, contain iodine?Did you know that table salt and many salt seasonings have iodine added to them?

Each year a few patients come in to my office with a history of shellfish allergy and have been told by a doctor, to avoid iodine contrast. Iodine contrast solution is used for various radiological tests (certain X-rays and Cat-scans). Some of these patients have never had iodine contrast while others have previously had exposure with no   reported problem.  Yet they've been advised to avoid iodine contrast solely on the basis of shellfish allergy.

Unfortunately the misconception that "shellfish allergy is directly associated with iodine or iodine contrast sensitivity" continues to surface in healthcare environments. The impact is regrettable for those who may have been denied or delayed in getting important diagnostic tests.

The prophylactic (preventive) treatment for iodine contrast hypersensitivity includes multiple doses of oral or injected steroids over a span of hours leading up to the infusion of the contrast, as well as antihistamine minutes before. Surprisingly, these measures may be unnecessary in patients who have never had a reaction to iodine contrast, even if they have history of shellfish allergy.

People avoiding shellfish solely on the basis of iodine contrast hypersensitivity may also be misinformed.  Allergists have attempted to explain how this misconception may have emerged through the years.

Shellfish, as well as other fish (fin fish) and seaweed, contain Iodine that has leached into seawater from the soil. Some of the iodine is retained in shellfish and fin fish, and can serve as a source of iodine through human consumption. We need iodine in order to have proper thyroid function. For this reason, many years ago, iodine was added to table salt and other seasonings as an additional source, particularly for inland dwellers with diets relatively low in seafood.

Decades ago doctors unjustifiably linked seafood allergy and iodine contrast reactions because of the presence of iodine in both entities.

In addition, at the time the incidence of both seafood allergy and iodine contrast reactions was on the rise. Thirty years ago iodine contrast reactions were more common because of the use of high ionic formulations. Currently, low ionic iodine contrast is used in most radiology departments. Since low ionic contrast has emerged, the frequency of contrast reactions has markedly decreased.

Bottom Line:

A history of shellfish allergy doesn't necessarily mean you are allergic or hypersensitive to iodine contrast. Any food allergy (including shellfish) increases the risk to reacting to iodine contrast, but the likelihood is very low.

A history of reacting to iodine does not necessarily mean you are allergic to shellfish. The iodine contrast does not cross-react with shellfish protein. On the other hand, you can be allergic to iodine and, in addition, have a shellfish allergy. If you have been avoiding shellfish for a long time solely on the basis of iodine contrast hypersensitivity, request an allergy consultation to evaluate this further.

If you have previously reacted to a topical iodine formulation (Povidine or Betadine), your risk of shellfish allergy is not increased. The report referenced below (second reference) explains that most skin reactions are the result of irritation and not an allergic mechanism.

It is important to discuss any history of intolerance to food, drugs (prescribed and over the counter) or topical agents with your doctor. Iodine contrast reactions, although rare, still occur and may be life-threatening. Risk of iodine contrast reaction is increased when there is a prior history of it.

Doctors would rather be safe than sorry, so prophylactic measures will probably continue to be ordered for iodine contrast infusions when there is a history of shellfish allergy. But consider this posting "food for thought."