Iodized salt is dosed at a rate to prevent goiters, which is caused by an extreme iodine deficiency. More than this minimal amount is beneficial.

There's a lot of research on iodine and comparing the traditional japanese diet to the standard American diet and the health outcomes of each. The papers vary in opinion and recommended dosage, so you can draw your own conclusions.

Also look how iodine is used in the body. What metabolic processes is is part of?