Characteristics: Iodine is a trace element that occurs naturally in food, is added to some types of salt and is available as a dietary supplement. 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 are critical determinants of metabolic activity. They are also essential for proper skeletal and central nervous system development in fetuses and infants. Iodine deficiency leads to goiter, an enlargement of the thyroid gland, which reflects the body's attempt to capture as much iodine as possible from the circulation. Iodine plays a role in the immune response and may have a beneficial effect in breast disease.

Absorption: Iodine in food and iodized salt is present in several chemical forms including sodium and potassium salts, inorganic iodine (I2), iodate and iodide (reduced form of iodine). Iodine is rarely found as an element, but rather as a salt; for this reason, it is referred to as iodide and not iodine. Iodide is rapidly and almost completely absorbed in the stomach and duodenum. Iodates are reduced in the gastrointestinal tract and absorbed as iodide. When iodide enters the circulation, the thyroid gland concentrates it in adequate amounts for thyroid hormone synthesis, and most of the remaining amount is excreted in the urine. A healthy adult has about 15-20 mg of iodine, of which 70-80% is contained in the thyroid gland. It is also found in high concentrations in the salivary, gastric and mammary glands (exclusively during pregnancy and lactation). Iodine uptake is regulated by thyroid-stimulating hormone.

Dietary supplements: In dietary supplements, iodine is often present as potassium iodide or sodium iodide. There are also supplements containing kelp, a seaweed that contains iodine. Humans have been found to absorb potassium iodide best (96.4%). Many multivitamin/mineral supplements contain iodine, often at 150 µg.

Natural sources: Seaweeds (such as kelp, nori, kombu and wakame) are one of the best food sources of iodine. Other good sources include fish, seafood, and eggs. Iodine is present in human breast milk and infant formula. Dairy products contain iodine. However, the amount of iodine in dairy products varies depending on whether the cows received iodine supplements and whether iodophor disinfectants were used to clean the cows and milk processing equipment. Plant-based drinks used as milk substitutes, such as soy and almond drinks, contain relatively small amounts of iodine. Most fruits and vegetables are poor sources of iodine, and the amount they contain is affected by the iodine content of the soil, fertilizer use, and irrigation. This variability affects the iodine content of meat and animal products because it affects the iodine content of the foods that animals consume. Unlike many other essential nutrients, the organic form of iodine in animal products has poor bioavailability, while the iodide salts found in the sea are almost completely absorbed.

Effect: Because of its important role in fetal and infant development and thyroid hormone production, iodine is a critical nutrient for proper health at all stages of life.
Deficiency: Iodine deficiency has numerous adverse effects on growth and development and is the most common preventable cause of intellectual disability in the world. During pregnancy and early childhood, iodine deficiency can cause irreversible changes. If iodine intake falls below about 10-20 µg/day/person, hypothyroidism occurs, a condition that is often accompanied by goiter. Goiter is usually the earliest clinical sign of iodine deficiency. In pregnant women, such iodine deficiency can cause deficits in the development of the nervous system and fetal growth retardation, as well as miscarriage and stillbirth. Chronic, severe iodine deficiency in utero causes cretinism, a condition characterized by intellectual disability, motor spasticity, stunted growth, delayed sexual maturation, and other physical and neurological abnormalities. Mild to moderate maternal iodine deficiency has also been associated with an increased risk of attention deficit hyperactivity disorder in children. In infants and children, even less severe iodine deficiency can cause neurodevelopmental deficits, such as somewhat lower than average intelligence as measured by IQ. Even in adults, mild to moderate iodine deficiency can cause deterioration of mental functions and work productivity. Chronic iodine deficiency may be associated with an increased risk of follicular thyroid cancer. The use of iodized salt is the most widely used strategy to control iodine deficiency. Deficiency can be caused by eating foods with goitrogens, which are substances that interfere with the absorption of iodine in the thyroid gland. Foods high in goitrogen include soy, cassava, and cruciferous vegetables (such as cabbage, broccoli, and cauliflower). Iron and/or vitamin A deficiency can also be goitrogenic (disrupting the thyroid gland and causing it to enlarge).

Recommended daily dose: adult: 150 µg for men, 150 µg for women (220 µg during pregnancy, 290 µg during breastfeeding).

Adverse effects: not described in normal doses.

Interactions: Soy inhibits iodine absorption and its high levels of isoflavones suppress T3 and T4 production. Selenium is essential for the proper functioning of thyroid hormones. ACE inhibitors, used to treat high blood pressure, may increase the risk of hyperkalemia (increased levels of potassium in the blood). Potassium-sparing diuretics also increase this risk.

Pregnancy: recommended to follow the recommended dosage, avoid excessive use.

Breastfeeding: safe in usual doses.

Toxicity: Chronic iodine intoxication occurs when iodide intake is approximately 2 mg per day or more. Excessive consumption of iodine can cause gastrointestinal irritation, abdominal pain, nausea, vomiting and diarrhea, cardiovascular symptoms and can induce hypo- and hyperthyroidism. Taking very high doses can lead to a metallic taste in the mouth, increased salivation, stomach irritation and the formation of acne-like skin lesions.


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