Iodine is a chemical element with the symbol I and atomic number 53. It is a halogen and is found naturally in several chemical forms, although elemental iodine is a relatively reactive substance rather than the form in which humans ordinarily obtain the nutrient. The name derives from a Greek word associated with violet colouring, reflecting the characteristic colour of iodine vapour. Naturally occurring iodine consists predominantly of the stable isotope iodine-127. Radioactive iodine isotopes also exist and have important medical and technological applications, but these should be distinguished from ordinary nutritional iodine. In human biology, iodine is an essential trace element because it is incorporated into thyroid hormones.
The principal biological function of iodine is to provide the iodine atoms required for synthesis of thyroid hormones. These hormones influence metabolic activity, growth, development, protein synthesis and the maturation of the nervous system. Thyroid hormone production occurs in the thyroid gland, where iodine is actively transported and incorporated into hormone precursors. The body consequently maintains specialised mechanisms for concentrating iodine in thyroid tissue.
Iodine requirements are particularly important during foetal development, infancy and childhood because thyroid hormones have major roles in brain and skeletal development. Iodine physiology is therefore not limited to adult metabolism; adequate iodine availability is essential throughout life, with particular developmental consequences when deficiency occurs during pregnancy or early childhood.
Dietary iodine is obtained from sources that vary considerably between countries. Seafood can be a significant source because marine environments contain iodine, while dairy foods may provide substantial amounts in populations where iodine-containing compounds enter the dairy production system. Eggs can also contribute. Iodised salt is an important public-health strategy in many countries because adding a controlled quantity of iodine to salt can increase iodine intake across a population.
The iodine content of plant foods is much more variable because it depends heavily on soil and environmental conditions. Consequently, a food category that is a reliable iodine source in one country may provide considerably less in another. This geographical and agricultural variability is one reason iodine nutrition is considered at the population level rather than inferred from food names alone.
Iodine is absorbed efficiently from many dietary sources, generally as iodide or other iodine-containing compounds, and is transported through the circulation. The thyroid actively concentrates iodine through a specialised transporter and converts it into forms used for hormone synthesis. Excess iodine is predominantly eliminated through urine, making urinary iodine measurements useful for assessing iodine intake at a population level.
A single urinary measurement, however, is not necessarily an accurate representation of an individual's long-term iodine status because urinary iodine varies with recent intake and hydration. Population surveys therefore commonly use distributions of urinary iodine measurements rather than interpreting an isolated value as a definitive diagnosis. The body also recycles iodine released during thyroid hormone metabolism, which contributes to maintaining supply.
Iodine deficiency reduces the availability of substrate for thyroid hormone production and can cause enlargement of the thyroid gland, known as goitre. More severe or prolonged deficiency can produce hypothyroidism and, during pregnancy and early development, can impair neurological development. Historically, iodine deficiency was widespread in areas with iodine-poor soils, and the recognition of its effects led to large-scale iodisation programmes. Iodine deficiency remains relevant in some populations despite major improvements in many countries. Excess iodine can also disturb thyroid function, particularly in susceptible individuals or populations with previous iodine deficiency. Thus, iodine nutrition follows a U-shaped conceptual pattern in which both inadequate and excessive exposure can be problematic.
Iodine is often confused with iodide, although the terms have different chemical meanings. Iodine refers to the element, while iodide refers to the negatively charged ion I− that commonly occurs in biological and dietary contexts. Nutritional iodine may also be present in organically bound compounds. Distinguishing these forms matters because chemical form affects absorption, transport and metabolism. Iodine should also not be confused with radioactive iodine used in medicine. Radioactive iodine isotopes have distinctive nuclear properties and can be administered under controlled medical circumstances for diagnostic or therapeutic purposes. Those applications are fundamentally different from dietary iodine nutrition even though they involve the same element.
Iodine has extensive medical, industrial and laboratory significance. Radioactive iodine isotopes are used because thyroid tissue naturally concentrates iodine, allowing certain isotopes to be used in diagnostic imaging or targeted treatment under specialist supervision. Iodine-containing compounds have also been used in antiseptic products and contrast materials, although individual formulations have different properties and safety considerations. Industrially, iodine compounds have applications in chemical synthesis and other specialised processes. These uses demonstrate that iodine's importance extends beyond nutrition, but the relevant chemical form and dose differ substantially between applications. Elemental iodine should not be treated as an ordinary dietary supplement because nutritional requirements concern controlled amounts of physiologically appropriate iodine compounds.
The key feature of iodine is its direct connection to thyroid hormone synthesis and therefore to metabolic regulation and development. Iodine deficiency is preventable in many circumstances through appropriate dietary strategies and population-level fortification, while excessive exposure can also produce thyroid disturbances. Geographic differences in food iodine content, variation in iodisation practices and the presence of both nutritional and radioactive forms make context essential when interpreting the term. Iodine is therefore simultaneously a chemical element, an essential nutrient, a public-health concern and a component of specialised medical technologies.
