Iodine is one of the main ingredients used to produce thyroid hormones.
The process of creating these hormones creates toxic byproducts that require selenium based enzymes to neutralize.
Antibodies can attack proteins involved in the production of thyroid hormones rendering them dysfunctional.
So if you body is lacking Iodine to make thyroid hormones then the toxic byproduct creation is low.
If all of a sudden you introduce Iodine and thyroid production goes up, the bodies need for selenium increases to cope with the byproducts.
If selenium is absent these byproducts can damage the thyroid which in turn triggers the antibody response which in turn reduces thyroid hormone production to get the damage caused by the toxic byproducts under control.