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.
(See also Overview of Minerals.)
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.
Blood tests to determine levels of thyroid hormones and thyroid-stimulating hormone (TSH) are done. Imaging tests may also be done.
People with excess iodine 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.