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Potential Risks of Excess Iodine Ingestion and Exposure:
Statement by the American Thyroid Association Public Health Committee

Volume 25, Number 2, 2015

Angela M. Leung,1,* Anca M. Avram,2 Alina V. Brenner,3 Leonidas H. Duntas,4 Joel Ehrenkranz,5
James V. Hennessey,6 Stephanie L. Lee,7 Elizabeth N. Pearce,7 Sanziana A. Roman,8
Alex Stagnaro-Green,9 Erich M. Sturgis,10 Krishnamurthi Sundaram,11
Michael J. Thomas,12 and Jason A. Wexler13
for the American Thyroid Association Public Health Committee

Dear Editor:

Iodine is amicronutrient required for normal thyroid function. In the United States, recommended daily allowances (RDA) for iodine intake are 150 lg in adults, 220–250 lg in pregnant women, and 250–290 lg in breastfeeding women (1,2).

The U.S. diet generally contains enough iodine to meet these needs, with common sources being iodized salt, dairy products, some breads, and seafood. During pregnancy and lactation, women require higher amounts of iodine for the developing fetus and infant.

The American Thyroid Association (ATA) recommends that women take a multivitamin containing 150 lg of iodine daily in the form of potassium iodide (KI) (3) during preconception, pregnancy, and lactation to meet these needs (4).

Ingestion of greater than 1100 lg of iodine per day (tolerable upper limits for iodine) (1) is not recommended and may cause thyroid dysfunction. During pregnancy and lactation, when the risk of excess iodine are primarily related to the fetus and newborn infant, the recommendations for the upper limit vary and range from 500–1100 lg of iodine daily (2).

In particular, infants, the elderly, pregnant and lactating women, and individuals with preexisting thyroid disease (such as autoimmune Hashimoto's disease, Graves' disease, nontoxic thyroid nodules, history of partial thyroidectomy, and other conditions) are susceptible to adverse effects of excess iodine intake and exposure (5).

The public is advised that many iodine, potassium iodide, and kelp supplements contain iodine in amounts that are up to a hundred times higher than the daily tolerable upper limits for iodine. TheATA advises against the ingestion of iodine and kelp supplements containing in excess of 500 lg iodine daily for children and adults and during pregnancy and lactation.

Longterm iodine intake in amounts greater than the tolerable upper limits should be closely monitored by a physician. There are only equivocal data supporting the benefit of iodine at higher doses than these, including a possible benefit for patients with fibrocystic breast disease (6). There is no known thyroid benefit of routine daily iodine doses in excess of the U.S. RDA

There are a limited number of medical conditions in which the short-term use of high amounts of iodine is indicated. Exceptions for the recommendations to not exceed the tolerable upper limits include closely monitored patients prescribed Lugol's solution or saturated solution of potassium iodide (SSKI) in their treatment of severe hyperthyroidism, such as thyroid storm and prior to surgery in patients with Graves' disease, and individuals in the vicinity of a nuclear power plant who are recommended to take KI in the event of a nuclear accident. SSKI is not indicated nor recommended in individuals with thyroid nodules.

Finally, patients receiving the large amounts of iodine in iodinated contrast dyes, as required for radiologic studies, should be monitored for iodine-induced thyroid dysfunction if risk factors are present.

Key points include:

  • Adequate iodine intake is required for normal thyroid function.
  • The recommended iodine intake in nonpregnant adults is 150 lg daily.
  • Pregnant and breastfeeding women should take a prenatal vitamin that contains 150 lg of potassium iodine daily.
  • Given a tolerable upper limit of 1100 lg iodine daily, ingestion of an iodine or kelp supplement containing in excess of 500 lg iodine daily should not be done.
  • Certain exceptions to these recommendations include those for specific medical conditions, which usually
    require only a limited number of doses for a short-term duration; such individuals should be closely monitored for thyroid dysfunction.

To download a PDF of this article (which includes references) please click here


Effect of small doses of iodine on thyroid function in patients with Hashimoto's thyroiditis residing in an area of mild iodine deficiency

European Journal of Endocrinology (1998) 139 23–28

W Reinhardt, M Luster, K H Rudorff, C Heckmann, S Petrasch, S Lederbogen, R Haase, B Saller, C Reiners, D Reinwein and K Mann

Objective: Several studies have suggested that iodine may influence thyroid hormone status, and perhaps antibody production, in patients with autoimmune thyroid disease. To date, studies have been carried out using large amounts of iodine. Therefore, we evaluated the effect of small doses of iodine on thyroid function and thyroid antibody levels in euthyroid patients with Hashimoto's thyroiditis who were living in an area of mild dietary iodine deficiency.

Forty patients who tested positive for anti-thyroid (TPO) antibodies or with a moderate to severe hypoechogenic pattern on ultrasound received 250 mg potassium iodide daily for 4 months (range 2–13 months). An additional 43 patients positive for TPO antibodies or with hypoechogenicity on ultrasound served as a control group. All patients were TBII negative.

Seven patients in the iodine-treated group developed subclinical hypothyroidism and one patient became hypothyroid. Three of the seven who were subclinically hypothyroid became euthyroid again when iodine treatment was stopped. One patient developed hyperthyroidism with a concomitant increase in TBII titre to 17 U/l, but after iodine withdrawal this patient became euthyroid again. Only one patient in the control group developed subclinical hypothyroidism during the same time period. All nine patients who developed thyroid dysfunction had reduced echogenicity on ultrasound. Four of the eight patients who developed subclinical hypothyroidism had TSH concentrations greater than 3 mU/l. In 32 patients in the iodine-treated group and 42 in the control group, no significant changes in thyroid function, antibody titres or thyroid volume were observed.

Small amounts of supplementary iodine (250 mg) cause slight but significant changes in thyroid hormone function in predisposed individuals.

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Study reported in International Council for the Control of Iodine Deficiency Disorders

ICCIDD Newsletter - Vol 7, Number 3, August 1991

Andreas Schuld tells us about this study, which is the only study in existence which investigated the levels of TSH in addition to T3 AND Reverse T3 in comparison with fluoride levels in water. This is an abstract:

“The significant differences in IQ among these regions suggests that fluoride can exacerbate central nervous lesions and somatic developmental disturbance caused by iodine deficiency. This may be in keeping with fluoride's known ability to cause degenerative changes in central nervous system cells and to inhibit the activities of many enzymes, including choline enzymes, causing disturbance of the nerve impulse (5). We found significant differences among the three areas, indicating that lack of iodine in children results in disturbance of the process of growth and ossification and that high fluoride intake can further disturb bone development (6,7). Also, the auditory threshold was significantly different among the three areas, with severe loss of hearing in high fluoride and low iodine areas. Severe iodine deficiency in early fetal life has adverse effects on the development and differentiation of the acoustic organ, and we suggest that high fluoride intake may also promote hearing loss.

In this study, we found that 69% of the children with mental retardation had elevated TSH levels. IQ and TSH were negatively correlated. Many investigators regard an elevated TSH in the presence of normal T4 and T3 levels as evidence for hypothyroidism that is subclinical but that can still affect the development of brain and cerebral function to some degree (6). Reverse T3 (rT3) is formed from T4 by 3-deiodination in peripheral tissue. The balance of active T3 and inactive rT3 in the serum reflects thyroid hormone economy. In high fluoride and low iodine areas, the rT3 value was 58 ng/dl (the normal value, 21 ng/dl), and the ratio of rT3/T3 was 2.91, significantly low. In areas of low iodine the rT3 value was 32 ng/dl, and the ratio of rT3/T3 was 5.8. It is possible that excess fluoride ion affects normal deiodination.