Highlights of the American Thyroid Association Symposium - Thyroid Health and the Environment: Threats and Effects

March 24, 2006; Washington, DC

Kenneth D. Burman, MD

Disclosures

December 12, 2006

In This Article

Iodine

A 1-day symposium entitled "Thyroid Health and the Environment: Threats and Effects" was sponsored by the American Thyroid Association, in cooperation with the American Association of Clinical Endocrinologists, to discuss the effects of the environment on the structure and function of the thyroid gland. There were many interesting and important topics reviewed, but this report focuses on the effects of iodine, perchlorate, and radiation.

Sufficient iodine intake is required for normal thyroid gland hormone synthesis and the maintenance of euthyroidism.[1] The World Health Organization (WHO) recommends a daily intake of at least 150 micrograms (mcg) of iodine for adults, 200 mcg for pregnant women, 90-120 mcg for children aged 2-11 years, and 50 mcg for infants younger than 2 years of age. It is thought that these minimum daily iodine intakes are sufficient to maintain normal thyroid homeostasis, whereas lower intakes may be associated with goiter and hypothyroidism and, perhaps, thyroid hormone deficiency in infants of mothers with iodine deficiency. Although less well characterized, tolerable upper limits of iodine exposure are probably approximately 1000 mcg/day.

In an early US national survey (National Health and Nutrition Examination Survey [NHANES I]; 1971-1974), the median urine iodine excretion was 320 mcg/day.[2] A follow-up survey showed decreasing urine excretion and, by deduction, intake. The NHANES III (1998-1994) analysis of 20,369 individuals aged 6-74 years found that the median urine iodide excretion was 145 mcg/day, and a short-term analysis from 2001 to 2002 showed it to be 161 mcg/day.[1,2] Because almost all of the iodide that is ingested is excreted in the urine, it is generally thought that the urine iodine measurement accurately reflects intake. In NHANES III, approximately 15% of women of childbearing potential, 7% of pregnant women, and 12% of adults had urine iodine values lower than 50 mcg/day. It is not known, of course, how consistent daily iodine intake or urine iodine measurements were in these subjects.

Pearce[3] and Pearce and colleagues[4] demonstrated that urine iodine excretion in 100 presumably healthy women living in the Boston, Massachusetts, area varied widely. When divided into quintiles, the lowest quintile had an estimated mean iodine urine concentration of 52 mcg/day, and the second quintile had a median concentration of 96 mcg/day. These values suggest that iodine intake in these subjects may be relatively insufficient, at least at the time measured.

Regardless of notions to the contrary, the greatest source of iodine in the United States is not from fish or iodized salt but iodine derived from dairy products and flour products.[3] The iodine concentration in milk is variable but comes from both udder contamination by iodine-containing antiseptics and the increased iodine content of cattle feed. In flour products, iodate is used as a preservative and conditioner. In a Boston survey of 20 different, commercially available bread preparations, 3 breads contained more than 313 mcg of iodine per slice, whereas the others contained 2-54 mcg of iodine per slice.[3,4] In common daily use by an individual consumer, it is difficult to accurately assess the iodine concentration of an individual dairy or flour product. It is estimated that only approximately 15% of daily iodine intake is derived from personal use of salt. Iodized salt contains about 400 mcg of iodine in 1 teaspoon, and salt used for commercial preservation typically does not contain iodine.[3,4] Pearce[3] and Pearce and colleagues[4] observed that various commercial preparations of milk had iodine concentrations between 60 mcg and 180 mcg per 250-mL sample. These estimates are interesting but should be interpreted with caution because they were performed only once, and a single, random urine iodine value may not represent oral intake over time.

It is important that breast milk contain a sufficient amount of iodine to allow normal thyroid hormone synthesis in infants. In 27 women living in the Boston area, the median breast milk iodine concentration was 157 mcg/L.[3] Given the estimated average breast milk intake of about 0.78 L/day, it is calculated that the average daily iodine intake by a breast-feeding infant is 162 mcg/day, with the minimum daily requirement being 110 mcg/day in infants up to 6 months of age.[3]

Of course, medications may also contribute significantly to iodine intake. Most important is amiodarone, an antiarrhythmic agent that contains about 75 mg/200-mg tablet. Radiocontrast agents, expectorants, and topical antiseptics also contain iodine.[3] Multivitamins available in the United States, even if labeled "prenatal," do not necessarily contain an adequate amount of iodine.

In summary, it appears that most adults in the United States have an adequate degree of iodine intake, but specific groups, such as pregnant and lactating women, need to take special care to ensure adequate iodine intake.[3]

Mild and moderate iodine deficiency is most commonly associated with an enlarged thyroid gland, which then may be associated with elevated thyroid-stimulating hormone (TSH) levels, especially when the iodine deficiency is severe and chronic. It is important for the somatic and neurologic development of the fetus and subsequent childhood development that appropriate maternal iodine intake be maintained before and during pregnancy. When clinically appropriate, a spot urine iodine determination will provide valuable information in regard to the sufficiency of maternal iodine intake.

As noted above, special consideration must be given to the iodine intake of pregnant and nursing women because of the potentially adverse effect that iodine deficiency may have on fetal and neonatal thyroid development. Inadequately treated maternal hypothyroidism is also associated with an increased chance of fetal loss.[5,6]

Haddow and colleagues[7] measured TSH in stored serum samples from 25,216 pregnant women and identified 62 women with an elevated TSH obtained during pregnancy; they were compared with 124 control women with normal TSH values in pregnancy during the same time period. The investigators subsequently studied the children of these woman with detailed neuropsychological tests when the children were between the ages of 7 and 9 years. The children of the 62 women with high serum TSH concentrations (ie, maternal hypothyroidism) had IQ scores on the Wechsler Intelligence Scale for Children that averaged 4 points lower than those of the children of the 124 matched-control women (P = .06). Forty-eight of the 62 hypothyroid women were apparently not treated with thyroid hormone during their pregnancies. The IQ scores of their children averaged 7 points lower than those of the 124 matched-control children (P = .005). This important study needs to be confirmed, and additional studies should be performed assessing serial thyroid function tests during pregnancy with subsequent serial neuropsychological testing of their progeny. Nonetheless, if maternal iodine deficiency can cause or aggravate relative maternal thyroid hormone deficiency, and if, in turn, maternal hypothyroidism can lead to impaired childhood development, it becomes extremely important to ensure sufficient iodine intake in women who are considering becoming pregnant or who are already pregnant. One practical consideration is to recommend that these women ingest an iodine-containing prenatal multivitamin and, in selected cases, to measure a random urine iodine.

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