IODINE 12.091 Medical Geology/Geochemistry: Element Report IAP 2006, 01/30/2006 Christopher Love Introduction If you have ever looked carefully at your commercial container of salt sitting in the cupboard, then you have noticed that it is iodized. Have you ever wondered what that means, or why it is mentioned? The simple answer to these questions is that a form of the element, iodine, is added to the salt in order to keep you healthy. More complex is the underlying reason behind many different types of food fortifications: medical geology. According to medical geology, humans are linked with the natural processes of the Earth by requiring certain levels of particular elements and compounds to maintain optimal health. Thus, iodine is added to salt in order to supply enough iodine in your diet and satisfy the link between iodine and optimal health. In order to understand why iodine is important for humans to consume, I will examine iodine’s chemical properties, where it occurs naturally, how it interacts in biological processes, its geologic properties, and the implications of this multidisciplinary analysis. Chemical characteristics Chemistry identifies iodine according to its chemical properties, such as the ability to react with other substances, physical properties at various temperatures, and color. Iodine---chemical symbol ‘I’---is the name given to element 53 on the periodic table of elements. It is a nonmetallic element that belongs to Group 17/VIIA of the periodic table (the halogen group). The color of iodine in its gaseous state is violet, which gives iodine its name: it comes from the Greek word for violet (‘iodes’) [Atkins and Jones pp 592]. Iodine has many other known 1chemical and physical properties: atomic weight is 126.9045 grams, melting point is 113.7 ˚C, boiling point is 184.4 ˚C, and atomic radius is 133.3 pm [Atkins and Jones pp 592]. As a solid, which is its normal form (at 25 ˚C and 1 atm), iodine has a blue-black color and sublimes easily [Atkins and Jones pp 592]. When it is dissolved in organic solvents, it produces solutions having a variety of colors; starch is a common indicator for the presence of iodine in solution by causing a solution of iodine to change color [Atkins and Jones pp 592]. If iodine touches the skin of a person, lesions can occur; furthermore, iodine vapor is intensely irritating to the eyes and mucus membranes [Atkins and Jones pp 592]. Even though iodine can be harmful if it is carelessly handled, it still has many practical uses: silver iodide is used in photography, and it is combined with alcohol and used as a disinfectant for wounds [Atkins and Jones, pp 592]. Natural Occurrence Where else does iodine exist outside of table salt? First of all, we encounter iodine in nature as iodide ions in brines and as an impurity in Chile saltpeter; Chile is the leading producer of iodine in the world (USGS 2). The main natural source of iodine is kelp: 2000 kg of seaweed produce about 1 kg of iodine [Atkins and Jones, pp 592]. In fact, Bernard Courtois is credited with the discovery of iodine when he “accidentally added concentrated sulfuric to the seaweed Fucus vesiculosus” [Fuge, pp 418]. What’s important to note is that Iodine is more naturally abundant near the sea; this fact will result in a link between iodine’s geology and biological consequences later on in the paper. In order to understand the link, I must first explain the biological role of iodine. Essentiality of Iodine: Iodine is an essential trace element, meaning that the human body needs iodine to function properly. Without enough of it, health problems can occur; with too much of it, toxic 2effects can appear. These observations result in a particular range in which iodine should be found in the body. According to Ron Fuge in Essentials of Medical Geology, the daily recommended dietary intake of iodine is estimated to be “110-130 µg/day for children under the age of 1, 90-120 µg/day for children aged 1-10, and 150 µg/day for adults and adolescents, with higher concentrations required during pregnancy and lactation” [Fuge, pp 426]. Humans need iodine because of the thyroid gland, which is located in the neck; anatomically, the thyroid gland consists of two lobes connected by a narrow isthmus, and it is composed of functional units called follicles [Lindh, pp 149]. The main function of the thyroid gland is to synthesize and store thyroid hormones: thyroxine (T4) and 3,5,3´-triiodothyronine (T3) [Lindh, pp 150]. Iodine’s role in the thyroid gland is to provide the raw material for hormone synthesis [Lindh, pp 150]. In fact, iodine contributes to 65% of T4 molecular size and 59% of T3 molecular size [Lindh, pp 150]. Thyroid hormones are important for “normal growth and development”; they control “the metabolic activity of many tissues through influences on the metabolism of carbohydrates, proteins, lipids, vitamins, nucleic acids, and ions” [Bernal and DeGroot, pp124-125]. The amount of iodine in the thyroid is another important indicator of its essentiality; normal adult thyroid weighs 20-25g and contains 8-10mg of iodine [Cherian and Nordberg, pp 189]. Iodine is selectively concentrated by the thyroid gland in amounts adequate for hormone synthesis. Most ingested iodine is reduced in the gastrointestinal tract and absorbed almost completely; the remaining iodine is excreted in the urine [Lindh, pp 150]. Deiodinases help to recycle iodine within the thyroid gland [Gershengor, pp 99]. Thyroid-stimulating hormone (TSH) is the major regulator of thyroid function [Cunnane, pp 119]. In response to concentrations of thyroid hormone, the pituitary gland secretes thyroid-stimulating hormone (TSH) to regulate thyroid function [Cunane, pp 120]. An elevated serum TSH concentration 3indicates primary hypothyroidism; in contrast, decrease in TSH concentration reflects hyperthyroidism [Lindh, pp 151]. Effects of Iodine Deficiency Hypothyroidism and hyperthyroidism are conditions caused by the amount of iodine in the body moving outside the range required for optimal health. Even though both conditions can occur, hypothyroidism is much more common in the world. Not consuming enough iodine can lead to Iodine Deficiency Disorders (I.D.D.). These disorders vary depending on the age of the person, but the two most common ones associated with iodine deficiency are goiter and cretinism. Goiter is a condition resulting from the thyroid gland