The thyroid gland contains most of the iodine in the body. Iodine in the thyroid gland is necessary for the formation of thyroid hormones.
Iodine occurs in seawater. A small amount of iodine from seawater enters the atmosphere and, through rain, enters ground water and soil near the sea.
In many areas, including the United States, table salt is fortified with iodine (in its combination form iodide) to help make sure people consume enough.
Iodine deficiency is rare in areas where iodine is added to table salt. However, the deficiency is common worldwide. People living far from the sea and at higher altitudes are at particular risk of iodine deficiency because their environment, unlike that near the sea, contains little, if any, iodine.
When iodine is deficient, the thyroid gland enlarges, forming a goiter, as it attempts to capture more iodine for the production of thyroid hormones. The thyroid gland becomes underactive and produces too little thyroid hormones (hypothyroidism). Fertility is reduced. In adults, hypothyroidism may cause puffy skin, a hoarse voice, impaired mental function, dry and scaly skin, sparse and coarse hair, intolerance to cold, and weight gain.
If pregnant women have iodine deficiency, the risk of miscarriage and stillbirth is increased. The fetus may grow slowly, and the brain may develop abnormally. Unless affected babies are treated soon after birth, a disorder that causes intellectual disability and short stature (cretinism) develops. Babies with cretinism may be deaf and mute. They may have birth defects and/or hypothyroidism.
Iodine deficiency is diagnosed based on blood tests indicating low levels of thyroid hormones or a high level of thyroid-stimulating hormone (TSH) or based on the presence of a goiter (only in adults).
Doctors do a blood test to check for hypothyroidism, including that due to iodine deficiency, in all newborns.
Imaging tests, such as ultrasonography or thyroid scanning, may be done to measure the thyroid gland and to evaluate any abnormalities.
Pregnant women often consume inadequate amounts of iodine. Thus, pregnant and breastfeeding women should take prenatal vitamins containing at least 150 micrograms of iodine daily.
Infants, children, and adults with iodine deficiency are treated with iodine supplements taken by mouth. Infants are also given supplements of thyroid hormone taken by mouth, for several weeks and sometimes throughout life. Children and adults may also be given thyroid hormone supplements.
Excess consumption of iodine is uncommon. It usually results from taking iodine supplements to treat a prolonged iodine deficiency. Sometimes people who live near the sea consume too much iodine because they eat a lot of seafood and seaweed and drink water that is high in iodine, as is common in northern Japan.
Consuming too much iodine usually does not affect thyroid function, but sometimes it does. It may cause the thyroid gland to become overactive and produce excess thyroid hormones (hyperthyroidism), particularly in people who used to consume too little iodine. However, sometimes excess iodine can decrease production of thyroid hormones (causing hypothyroidism). As a result, the thyroid gland enlarges, forming a goiter. (Goiters can form when the thyroid gland is underactive or overactive.)
If people consume very large amounts of iodine, they may have a brassy taste in their mouth and produce more saliva. Iodine can irritate the digestive tract and cause a rash.
Doctors suspect hyperthyroidism or hypothyroidism due to excess iodine based on symptoms, particularly in people who report taking iodine supplements.
Blood tests to determine levels of thyroid hormones and thyroid-stimulating hormone (TSH) are done. Imaging tests may also be done.
People are advised to use salt that is not fortified with iodine and to reduce their consumption of foods that contain iodine, such as seafood, seaweed, yogurt, and milk.
If people have hypothyroidism due to consuming too much iodine, consuming less iodine often cures the disorder, but some people must take thyroid hormones for the rest of their life.
If people have hyperthyroidism, drugs may be needed to help the thyroid gland function normally.