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.

Aside from its industrial applications, selenium is an essential trace nutrient. A deficiency in selenium is a severe obstacle in areas of the world where the soil contains little selenium such as the case in the United Kingdom, Australia, New Zealand and some areas of China among others. It has been estimated that over 1 billion people have a low selenium status.