The children of women who experienced hypothyroidism due to iodine deficiency in early pregnancy are more likely to have attention deficit–hyperactivity disorder (ADHD) than children of women who had adequate iodine intake during pregnancy, according to the Journal of Clinical Endocrinology and Metabolism (2005;89:6054–60).

ADHD is a disorder characterized by impulsiveness, difficulty controlling body movements, and difficulty maintaining attention and focus. ADHD is primarily a childhood disorder, and is more common in boys than in girls. It is estimated that 3 to 6% of school-aged children in the United States have ADHD, and it has become clear that the disorder can persist into adulthood. The cause is unknown but is believed to involve environmental, neurological, and hormonal factors. People with ADHD may have difficulties in school, at work, and in their relationships.

Iodine is a trace mineral found in seafood and in vegetables grown in soils that contain iodine. Most of the iodine in the body is stored in the thyroid gland and is used to make thyroid hormone. Iodine deficiency is one cause of low thyroid hormone levels, or hypothyroidism, the symptoms of which can include fatigue, easy weight gain, and depression. Adults need 150 mcg of iodine per day for normal thyroid function, but the requirement is higher in pregnant and breast-feeding women. People living in regions with iodine-poor soils are at risk of iodine-deficiency hypothyroidism, particularly during pregnancy. In the United States, iodine is added to commercial table salt and iodine deficiency is therefore less common than in some other countries. In recent years, however, health-conscious Americans have been adding less salt to their food and iodine deficiency rates have been increasing. Preliminary reports suggest that the prevalence of ADHD might be higher in regions where iodine levels in the soil are low.

The current study examined the effect of iodine deficiency during early pregnancy on the risk of ADHD in children. Sixteen children born to mothers in a region of Italy known to have iodine-poor soil were compared with 11 children born to mothers in a region known to have sufficient soil iodine. The mothers were tested for thyroid function at approximately 8 weeks, 13 weeks, and 20 weeks of gestation (the period from conception to birth is usually 40 weeks). The children were evaluated using behavioral, psychological, and neurological testing between the ages of 18 and 36 months, and between the ages of 8 and 10 years. An intelligence test (IQ test) was taken and questionnaires were filled out by parents and teachers at the second evaluation.

Hypothyroidism was detected at 20 weeks of gestation in 50% of the mothers from the iodine-poor region, and the children of all but one of these mothers were later diagnosed with ADHD (88%); none of the mothers from the iodine-sufficient region had hypothyroidism at 20 weeks. Sixty-nine percent of the children of mothers in the iodine-poor region were found to have ADHD at the second evaluation, while none of the children of mothers in the iodine-sufficient region had ADHD. Furthermore, scores on IQ tests were significantly lower in the children from the iodine-poor region than in those from the iodine-sufficient region, and this difference was more pronounced in children of mothers who had hypothyroidism during pregnancy.

The results of this study suggest that maternal hypothyroidism in early pregnancy can be a cause of ADHD in children. Other neurological disorders have previously been linked to maternal hypothyroidism, but this is the first study to examine the relationship between maternal hypothyroidism due to iodine deficiency and ADHD. More research is needed to confirm these findings, but in the meantime, pregnant women should be encouraged to use a nutritional supplement with iodine, especially if they do not consume fish or sea vegetables, and routine thyroid testing in early pregnancy should be considered in regions where soils are known to lack iodine.

Maureen Williams, ND, received her bachelor’s degree from the University of Pennsylvania and her Doctorate of Naturopathic Medicine from Bastyr University in Seattle, WA. She has a private practice in Quechee, VT, and does extensive work with traditional herbal medicine in Guatemala and Honduras. Dr. Williams is a regular contributor to Healthnotes Newswire.

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