Iodine excess

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Iodine excess is uncommon and is caused by consuming too much of the mineral iodine, which can sometimes affect thyroid function.
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.
(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.