There is a well established myth that fish and shellfish allergy is linked to the iodine content of fish and that iodine allergy and shellfish allergy are interrelated. This is untrue.
Shellfish such as shrimp, prawn, crab and lobster are crustaceans (crawl around) and have a protein in their flesh that can cause allergies, they contain too little iodine to cause any problems. Mussels, clams and squid are members of the mollusc (snail) family and a protein in their flesh can also cause food allergies, but they do not contain enough iodine to be a problem. Shellfish can ingest a nasty flagellate organism which causes food poison and a toxic reaction with vomiting, diarrhoea and flushing (this is not an allergy).
Iodine allergy is unrealated to shellfish but iodine can cause a local skin reaction when exposed (contact dermatitis) or a more severe anaphylactic reaction when radiocontrast dye is injected into a vein during invasive radiological examinations such as an isotope scan or angiogram used for investigation of kidney, heart, bone and brain conditions. Many doctors dont realise this seafood myth and will still ask if you are allergic to shellfish before giving radiocontrast.
The common joint supplement Glucosamine taken to help arthritis symptoms, although processed from shellfish extract (crab, shrimp and lobster shells), can safely be taken by people with shellfish allergies according to the Mayo Clinic in the USA.