Iodine is a crucial micronutrient that is essential for life. This essential trace element for humans and animals is required for the thyroid gland to produce the thyroid hormones, thyroxine, T4, and triiodothyronine, T3. These hormones regulate development of the body and brain, growth, and metabolism across the lifespan. Deficiency can lead to severe developmental damage in children (cretinism), thyroid swelling (goitre) and thyroid dysfunctions with fatigue, weight gain, and cognitive deficiencies, also known as iodine deficiency disorders, which may in turn influence longevity. While iodine is present in small amounts in seawater, seaweed has historically been a rich source of iodine. Most inland areas have iodine-poor soils, but iodine may be present in groundwater in some areas depending on the aquifer source rock. This contributes to variations in population iodine intake across geography. Still, iodine intake is often insufficient in many populations globally and many countries have introduced iodine fortification programs to combat iodine deficiency disorders and promote health. The importance of an adequate iodine intake on human health is mediated largely by its essential importance for thyroid hormone production, and iodine intake patterns across the human lifespan may have important implications for promoting longevity.