Is it correct that the Japanese have among the world's largest daily intake of Iodine due to consumption of foods containing a high-iodine sea weed?
Do I remember correctly that they consume around 10mg per day?
If most of your ancestors lived in a part of the world where people did not consume that much iodine, maybe your body would feel better on lower doses?
Maybe your body was fine during a loading phase but the dose became too much over time.