Evidence Level
Strong
Iodine's role in thyroid function is among the most well-established in nutrition science. The WHO estimates that 2 billion people worldwide have insufficient iodine intake. Universal salt iodization remains one of the most successful public health interventions globally. Zimmermann's 2009 Endocrine Reviews paper provides a comprehensive overview demonstrating that iodine deficiency is fully preventable with adequate intake. However, excess iodine (>1,100mcg daily) can paradoxically cause thyroid dysfunction, particularly in those with pre-existing autoimmune thyroid disease (Hashimoto's).