Iodine is an element found naturally in the earth, primarily in ocean waters. Soil in many areas of the world is relatively iodine deficient, which explains the fact that iodine deficiency is common worldwide. Iodine is necessary for normal thyroid function since it's used to make thyroid hormone. Thyroid hormone is made from iodide, the water-soluble ion of iodide, and tyrosine, an amino acid. Thyroid hormone is vital for regulating metabolism, the rate at which the body metabolizes energy and produces heat. During infancy, childhood and adolescence, it regulates growth and maturation of organs like the brain and reproductive system. Deficiency of thyroid hormone during infancy and childhood leads to delayed growth and mental retardation. Worldwide, iodine deficiency is the leading cause of mental retardation in children. It's important that pregnant women get enough iodine so their fetus develops normally.
During adulthood, iodine deficiency leads to a condition called hypothyroidism, or an underactive thyroid, since the thyroid gland can't make enough thyroid hormone due to lack of iodine. One sign of an underactive thyroid gland is an enlarged thyroid gland or goiter. Adults with hypothyroidism also commonly have other symptoms such as fatigue, weight gain, sensitivity to cold, dry skin, constipation and depression. Iodine deficiency has been linked with infertility, and some research suggests it may increase the risk of certain types of cancer, especially thyroid cancer and breast cancer.
Iodine is naturally found in some foods including seaweed, saltwater fish with smaller quantities present in dairy products and eggs. Plants that grow in iodine rich soil are also a good source of iodine. Because iodine isn't found in many foods and soil may not have adequate iodine levels, iodine is added to table salt to reduce the risk of deficiency. Most people get their iodine from iodized salt rather than food sources. Getting enough dietary iodine is important for supporting thyroid function and avoiding a slowdown in metabolism that can lead to weight gain.
Iodine is also a disinfectant that can be used to purify water. There are portable iodine kits available that can be used in the field - when camping or hiking. Iodine kills most of the common waterborne pathogens that can cause illness. Iodine also has applications in the medical field. In case of nuclear disaster, giving potassium iodide tablets block iodide-131 from nuclear fallout from being taken up by the thyroid gland. Derivatives of iodine are used as radiocontrast agents for certain types of imaging studies like CT. Iodine-131 is also used to treat a condition called thyrotoxicosis due to an overactive thyroid and some types of thyroid cancer.
A type of iodine compound called iodopropynyl butylcarbamate or IPBC is added to some cosmetic and personal care and home products as a preservative to block the growth of pathogens that would shorten the shelf-life of the product. Because iodine stops the growth of bacteria, a form of iodine called povidone-iodine can be applied topically to cuts and scrapes to reduce the risk of infection. It should be used with caution since some research suggests iodine may delay tissue healing. Areas of the skin that are treated with iodine shouldn't be bandaged since this can lead to a burn.
It's important to get enough dietary iodine for health, but people with acne should be aware that some studies have linked iodine in dairy products with acne outbreaks. Milk in some areas of the world including the United States and Europe contains significant amounts of iodine since cows are given iodine-enriched feed to reduce their risk for infection. It's not clear whether iodine is the component in milk that worsens acne, but some experts believe it is. It's also possible that hormones in milk are the culprit.
Iodine is important for health but taking large amounts may be unsafe. Adults should avoid taking more than 1100 micrograms per day unless directed by a doctor. Taking large amounts can cause stomach upset, sore gums, a metallic mouth taste, mouth burning, diarrhea and skin problems. People who use it topically can experience skin irritation, itching and skin redness, although this is relatively uncommon. As a preservative in skin care products, iodine is present in low concentrations and shouldn't cause side effects.