A dietary deficiency of iodine is the single most significant cause of preventable brain damage and mental retardation. A lack of iodine is one of the most common nutritional deficiencies in the world.

Iodine, in combination with the amino acid tyrosine, link together to manufacture thyroid hormones. Thyroxine (T4) is much more abundant. However, it is not as biologically active as triiodothyronine (T3). Thyroid hormone speeds up metabolism and increases basal metabolic rate, in addition to controlling the rate of oxygen utilisation and releasing energy from energy-producing nutrients.