Iodine Safety

Acute toxicity

Acute iodine poisoning is rare and usually occurs only with doses of many grams. Symptoms of acute iodine poisoning include burning of the mouth, throat, and stomach, fever, nausea, vomiting, diarrhea, a weak pulse, and coma.

Iodine excess

It is rare for diets of natural foods to supply more than 2,000 mcg of iodine/day, and most diets supply less than 1,000 mcg/day. People living in the northern coastal regions of Japan, whose diets contain large amounts of seaweed, have been found to have iodine intakes ranging from 50,000 to 80,000 mcg (50-80 mg) of iodine/day.

In iodine deficiency: Iodine supplementation programs in iodine-deficient populations have been associated with an increased incidence of iodine-induced hyperthyroidism (IHH), mainly in older people and those with multinodular goiter. Iodine intakes of 150-200 mcg/day have been found to increase the incidence of IHH in iodine-deficient populations. Iodine deficiency increases the risk of developing autonomous thyroid nodules that are unresponsive to the normal thyroid regulation system (see Functions), resulting in hyperthyroidism after iodine supplementation. IHH is considered by some experts to be an iodine deficiency disorder. In general, the large benefit of iodization programs outweighs the small risk of IHH in iodine-deficient populations.

In iodine sufficiency: In iodine-sufficient populations (e.g., the U.S.), excess iodine intake is most commonly associated with elevated blood levels of thyroid stimulating hormone (TSH), hypothyroidism, and goiter. Although a slightly elevated TSH level does not necessarily indicate inadequate thyroid hormone production, it is the earliest sign of abnormal thyroid function when iodine intake is excessive. In iodine-sufficient adults, elevated TSH levels have been found at iodine intakes between 1,700 and 1,800 mcg/day. In order to minimize the risk of developing hypothyroidism, the Food and Nutrition Board (FNB) of the Institute of Medicine set a tolerable upper level of intake (UL) for iodine at 1,100 mcg/day for adults. Very high (pharmacologic) doses of iodine may also produce thyroid enlargement (goiter) due to increased TSH stimulation of the thyroid gland. Prolonged intakes of more than 18,000 mcg/day (18 mg/day) have been found to increase the incidence of goiter. The UL values for iodine are listed by age group in the table below. The UL is not meant to apply to individuals who are being treated with iodine under medical supervision.

Tolerable Upper Intake Level (UL) for Iodine
Age Group	UL (mg/day)
Infants 0-12 months	Not possible to establish*
Children 1-3 years	200 mcg/day
Children 4-8 years	300 mcg/day
Children 9-13 years	600 mcg/day
Adolescents 14-18 years	900 mcg/day
Adults 19 years and older	1,100 mcg/day (1.1 mg/day)

*Source of intake should be from food and formula only.

Individuals with increased sensitivity to excess iodine intake: Individuals with iodine deficiency, nodular goiter, or

autoimmune thyroid disease may be sensitive to intake levels considered safe for the general population and may not be protected by the UL for iodine intake. Children with cystic fibrosis may also be more sensitive to the adverse effects of excess iodine.

Excess iodine and thyroid cancer: Observational studies have found increased iodine intake to be associated with an increased incidence of thyroid papillary cancer. The reasons for this association are not clear. In populations that were previously iodine deficient, salt iodization programs have resulted in relative increases in thyroid papillary cancers and relative decreases in thyroid follicular cancers. In general, thyroid papillary cancers are less aggressive and have a better prognosis than thyroid follicular cancers.

Drug interactions

Amiodarone, a medication used to prevent abnormal heart rhythms, contains high levels of iodine and may affect thyroid function. Medications used to treat hyperthyroidism, such as propylthiuracil (PTU) and methimazole may increase the risk of hypothyroidism. The use of lithium in combination with pharmacologic doses of potassium iodide may result in hypothyroidism. The use of pharmacologic doses of potassium iodide may decrease the anticoagulant effect of warfarin (coumarin).

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