Iodine is an essential element that is required for thyroid hormone production.

The measurement of iodine serves as an index of adequate dietary intake and iodine overload, particularly from iodine-containing drugs such as Amiodarone.

Sources:

Iodine is a trace element that is naturally present in some foods, added to others, and available as a dietary supplement.

The earth’s soils contain varying amounts of iodine, which in turn affects the iodine content of crops. In some regions of the world, iodine-deficient soils are common, increasing the risk of iodine deficiency among people who consume foods primarily from those areas. Salt iodization programs, which many countries have implemented, have dramatically reduced the prevalence of iodine deficiency worldwide.

Iodine and thyroid hormones:

Iodine is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Thyroid hormones regulate many important biochemical reactions, including protein synthesis and enzymatic activity, and are critical determinants of metabolic activity.

They are also required for proper skeletal and central nervous system development in fetuses and infants. 

Thyroid function is primarily regulated by thyroid-stimulating hormone (TSH), also known as thyrotropin. It is secreted by the pituitary gland to control thyroid hormone production and secretion, thereby protecting the body from hypothyroidism and hyperthyroidism. TSH secretion increases thyroidal uptake of iodine and stimulates the synthesis and release of T3 and T4.

In the absence of sufficient iodine, TSH levels remain elevated, leading to goiter, an enlargement of the thyroid gland that reflects the body’s attempt to trap more iodine from the circulation and produce thyroid hormones.

Other functions of iodine:

Iodine may have other physiological functions in the body as well. For example, it appears to play a role in immune response and might have a beneficial effect on mammary dysplasia and fibrocystic breast disease.

Absorption:

Iodine in food and iodized salt is present in several chemical forms including sodium and potassium salts, inorganic iodine, iodate, and iodide, the reduced form of iodine. Iodine rarely occurs as the element, but rather as a salt; for this reason, it is referred to as iodide and not iodine. Iodide is quickly and almost completely absorbed in the stomach and duodenum. Iodate is reduced in the gastrointestinal tract and absorbed as iodide. When iodide enters the circulation, the thyroid gland concentrates it in appropriate amounts for thyroid hormone synthesis and most of the remaining amount is excreted in the urine.