Iodine | The nutritional source
Iodine is an essential trace element not produced by the body and must therefore be obtained through food or supplements. It is found naturally in some foods and is added to supplements and some salty seasonings. Iodine is needed to make the thyroid hormones thyroxine and triiodothyronine, which help in the creation of proteins and enzymatic activity, as well as in the regulation of normal metabolism. Without enough iodine, these thyroid hormones do not work properly and can result in underactive or overactive thyroid gland causing hypothyroidism and hyperthyroidism medical condition with various negative side effects in the body.
RDA: The recommended dietary allowance for iodine is 150 micrograms (mcg) per day for adult men and women 19 years of age and older, and 220 and 290 mcg per day for pregnant and lactating women, respectively. [1,2]
UL: A tolerable upper intake level (AMT) is the maximum daily dose unlikely to cause unwanted side effects in the general population. The UL for iodine for adults 19 years of age and older and pregnant and lactating women is 1,100 mcg per day.
Iodine and health
Infant and Child Health
The production of thyroid hormones increases during pregnancy, which necessitates higher intakes of iodine. The fetus and infant need enough iodine for normal physical growth and brain development. Breast milk contains iodine, although the amount depends on the mother’s diet, and many infant formulas contain iodine. However, not all prenatal multivitamins contain iodine. The American Academy of Pediatrics recommends that prenatal and lactating women take an iodine supplement using iodized salt.  High dose iodine supplements are not always recommended, as they can cause thyroid problems in some women.
Moderate to severe iodine deficiency in children can lead to lower IQ and stunted growth. Iodine supplementation in these children appears to improve both their physical and mental development.
Hypothyroidism and hyperthyroidism
These conditions occur when the body produces too little (hypothyroidism) or too much (hyperthyroidism) of thyroid hormones. An early sign of hypothyroidism is having a high blood level of thyroid stimulating hormone (TSH). This can happen with low iodine intake. It can also occur in people with a history of iodine deficiency who take large doses of iodine such as in supplements.  Goiter, a lump in the front of the neck caused by an enlarged inflamed thyroid gland, can result from hypothyroidism, hyperthyroidism, or excessive iodine intake (usually involves taking very high doses of over 18,000 mcg per day for an extended period).  Likewise, the goitrogens in some cruciferous vegetables can prevent the absorption of iodine, which can then interfere with thyroid function and cause goiter. Those who already have iodine deficiency or hypothyroidism are the most susceptible.
Iodine is found in soil and the ocean, the amount of which varies and will affect the amount of minerals in a food. Iodine is found primarily in animal protein foods and seafood, and to a lesser extent in fortified foods such as bread, cereals and milk.
Signs of deficiency and toxicity
Iodine regulates metabolism, the conversion of energy obtained from food into energy to help cells function and grow. Iodine deficiency can therefore prevent normal growth and development. This is especially dangerous in pregnant women and infants, in whom miscarriage, stillbirth, growth retardation and cognitive impairment (difficulty reading, writing, speaking, problem solving, social skills) can occur. In adults, an iodine deficiency of less than 10-20 mcg per day can lead to insufficient production of thyroid hormones, called hypothyroidism, which disrupts normal metabolic functions such as the regulation of heart rate, body temperature and body weight.  A lump or swelling in the neck, called a goiter, often accompanies hypothyroidism. Other signs of hypothyroidism include:
- Fatigue, lethargy
- Cold sensitivity
- Dry skin and hair
- Weight gain
People at risk for iodine deficiency include those who do not use iodized salt or supplements containing iodine, pregnant women, vegans who do not eat foods of animal origin and those who live in areas with low iodine content in the soil (eg mountainous regions).
High iodine intakes are generally well tolerated by most healthy people and do not cause problems.  This has been observed in countries like Japan and Korea which regularly consume seaweed rich in iodine.  But some people with autoimmune thyroid disease or who have a history of chronic iodine deficiency may be sensitive to supplemental iodine intake, causing iodine-deficient conditions like hypothyroidism and goiter. [2,4] Too much iodine can also lead to excess production of thyroid hormones, causing hyperthyroidism; Signs of this disease are increased metabolism which promotes weight loss, fast or irregular heartbeat, hand tremors, irritability, fatigue and sweating. Sometimes even a slight increase in dietary iodine above the RDA can cause iodine-induced hyperthyroidism in susceptible people. 
Some epidemiological studies have shown that high seaweed consumption is associated with an increased risk of certain types of thyroid cancer, especially in postmenopausal women, but the exact mechanism is not clear. [2,3]
Excessive iodine intake can result from the use of high dose supplements or from overconsumption of certain algae and salts containing iodine. Severe iodine poisoning is rare, but symptoms include fever; Stomach pain; nausea; vomiting; a burning sensation in the mouth, throat and stomach; and even a coma.  Children, infants, the elderly, and people with existing thyroid disease are particularly vulnerable to iodine toxicity and iodine-induced hypothyroidism and hyperthyroidism. [3,4]
Did you know?
- In the United States, people get most of their dietary iodine from salt and iodized milk.
- Iodine supplements can interact with certain blood pressure medications and diuretics, including lisinopril, spironolactone, and amiloride, causing a dangerous buildup of potassium in the blood called hyperkalemia.
- Iodine is an ingredient in contrast agents that a person may take before having an x-ray or computed tomography (CT) scan. It helps absorb the rays so that clearer images of the body’s organs can be seen.
- Micronutrient Institute of Medicine Expert Group. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium and Zinc . Press of the National Academy, Washington, DC, 2001.
- US Department of Health and Human Services. Iodine Fact Sheet for Healthcare Professionals. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/ Accessed 09/20/21.
- Murai U, Yamagishi K, Kishida R, Iso H. Impact of algae consumption on health. European Journal of Clinical Nutrition. 2021 Jun; 75 (6): 877-89.
- Farebrother J, Zimmermann MB, Andersson M. Excessive iodine intake: sources, evaluation and effects on thyroid function. Annals of the New York Academy of Sciences. 2019 Jun; 1446 (1): 44-65.
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