I'm no expert but the answer is easy. Just take Iodide and add it to water and let it sit open to the air. The redox potential for the reaction of Iodide to Iodine is sufficient to enable Iodide to be oxidized to Iodine by ambient oxygen, as you will observe after the passage of a little time in the aqueous solution.
Then, consider that the blood is an oxygen-rich environment, and ask yourself what happens to a fraction of the iodide when present in blood......