What it's actually good for
Iodine is a trace mineral that is literally built into thyroid hormones — without it, the thyroid cannot produce T3 or T4, leading to hypothyroidism, goiter, fatigue, and weight gain. Iodine deficiency remains the most common preventable cause of intellectual disability worldwide, though iodized salt programs have largely eliminated severe deficiency in developed nations. Mild deficiency may still occur in those avoiding iodized salt, dairy, and seafood. The key nuance is that both too little and too much iodine disrupt thyroid function — making this a nutrient where the target range matters. Testing iodine status (urinary iodine) is more useful than empirical supplementation, particularly for those with thyroid autoimmunity.