You’re not allergic to iodine. That would be a fatal condition, as iodine is an essential element for human life. There is a popular notion that allergies to iodine-containing foods and products such as shellfish, radiologic contrast solution (for CT scans and angiograms) and surgical prep solutions like betadine represent an iodine allergy, which is false. The actual allergy in each case is to a unique component such as a muscle protein in shellfish or a carrier molecule for the contrast and prep solutions.
That is what the doctor told me (that I have an iodine allergy). Furthermore, he told me that it is not all shrimp — but the water that the shrimp was swimming in. All I know is that I break out in itchy hives if I eat shrimp (and he was right about that — depends upon where the shrimp came from, but I have no way of knowing in advance), have a contrast scan using iodine, or have batadine applied to my skin. All I know is that the reactions get worse the older I get, so I avoid all of those situations. I suppose the doctor was just being simplistic in his explanation, but no other medical professional has ever argued about it.