Diseases & Conditions



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 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

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—see Thyroid Gland Disorders: Hypothyroidism ). The person's IQ may be decreased. 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, mental retardation with short stature (cretinism) develops. Babies may have birth defects or hypothyroidism.

Did You Know...

Lack of iodine during pregnancy increases the risk of miscarriage, stillbirth, and mental retardation and birth defects in the baby.

Diagnosis and Treatment

Iodine deficiency is diagnosed based on blood tests indicating low levels of thyroid hormones or a high level of thyroid-stimulating hormone (TSH) or on the presence of a goiter (only in adults). The amount of iodine in urine is measured. The lower the amount, the more severe the deficiency. Imaging tests, such as ultrasonography or thyroid scanning, may be done to measure the thyroid gland and to evaluate any abnormalities.

Treatment consists of iodine supplements, taken by mouth. Infants may also require supplements of thyroid hormone, sometimes throughout life.

Iodine Excess

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 usually does not affect thyroid function, but sometimes does. It may cause the thyroid gland to become overactive and produce excess thyroid hormones (hyperthyroidism—see Thyroid Gland Disorders: Hyperthyroidism ). As a result, the thyroid gland enlarges, forming a goiter. (Goiters can also form when the thyroid gland is underactive.) 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.

Consuming too much iodine may also make the thyroid gland become underactive (hypothyroidism), especially if the thyroid gland had been underactive previously (for example, in people with Hashimoto's thyroiditis—see Thyroid Gland Disorders: Hashimoto's Thyroiditis ).

Diagnosis and Treatment

Doctors suspect hyperthyroidism or hypothyroidism due to excess iodine based on symptoms, particularly in people who report taking iodine supplements, who live near the sea, or who consume a lot of seaweed or seafood. 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. 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.

Last full review/revision August 2008 by Larry E. Johnson, MD, PhD

Source: The Merck Manual Home Edition