Allergies to shellfish have nothing to do with allergies to iodine, even though there are some urban legends about a connection between the two. Common belief was that people who were allergic to shellfish also had to stay away from iodine and the radiocontrast dye used in lab tests. However, there's no medical evidence to support the belief.
Obviously, people with shellfish allergies do have to stay away from shellfish and products made with shellfish. Other products that people with shellfish allergies should avoid include calcium coral, which is a supplement made from coral reefs, and possibly glucosamine, which is used to treat arthritis and is made from crab, lobster and shrimp shells. The other main concern for people with shellfish allergies is cross-contamination. When you eat at a restaurant -- particularly a seafood restaurant -- it's important to check what ingredients go into your meal and whether it's cooked in the same pan or oil as shellfish.