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Many factors have a role in the neuropsychological development of children, apart from maternal thyroid function. We are not given details of the women's marital status after delivery, even though parental separation and divorce or serial monogamy on the part of one parent can impair a child's development.2,3 The effect of siblings, though variable, should also be considered2,4; 73 percent of the women who had hypothyroidism during pregnancy were multiparous, as compared with 65 percent of the control women and 51 percent of the remaining cohort of pregnant women. Maternal thyroid status may be important in the development of a healthy child, but on the basis of the data presented by Haddow et al., it is not clear that widespread screening is justified.
Christian Herzmann
Leipzig University
04103 Leipzig, Germany
James K. Torrens, M.R.C.P.
Seacroft Hospital
Leeds LS14 6UH, United Kingdom
References
In those studies, low maternal serum thyroxine or free thyroxine concentrations in the first or second trimester were correlated with poor postnatal neuropsychological development. During the first trimester, maternal thyroxine is the only source of thyroid hormone for the fetus. Normal maternal serum triiodothyronine concentrations, which mitigate hypothyroidism in pregnant women, do not prevent poorer cognitive development in their infants.2 In preterm infants, who are prematurely deprived of maternal thyroxine and iodine, poor cognitive development and an increased risk of cerebral palsy are also correlated with the presence of perinatal hypothyroxinemia, not with high serum thyrotropin concentrations,4 indicating the continuing importance of thyroxine for later stages of brain development.
As Utiger noted in the accompanying editorial,5 an inadequate iodine intake may be a crucial factor contributing to the frequency of hypothyroidism during pregnancy, and it is also a crucial determinant of relative hypothyroxinemia. In Brussels, Belgium (with a median urinary iodine excretion of 56 µg per liter), 30 percent of pregnant women had low serum free thyroxine values in the first trimester and 2.3 percent had high serum thyrotropin values.6 Even in Madrid, an area where mild iodine deficiency is prevalent (median urinary iodine excretion, 90 µg per liter), 20 percent of pregnant women had first-trimester serum free thyroxine values below the 10th percentile of values for women with adequate iodine intake. These data support Utiger's recommendation that measures be taken to ensure that pregnant women have an adequate intake of iodine (>200 µg per day). Thyroxine therapy should be limited to pregnant women who have persistent hypothyroxinemia.
Gabriella Morreale de Escobar, Ph.D.
Francisco Escobar del Rey, M.D., Ph.D.
Instituto de Investigaciones Biomédicas Alberto Sols
28039 Madrid, Spain
References
Sapporo has a population of 1.8 million, and 16,000 infants are born every year. Our screening program for thyroid function in early pregnancy was initiated in June 1986 and consists of measurements of serum thyrotropin, free thyroxine, and antithyroid peroxidase and antithyroglobulin antibodies in blood specimens collected on filter paper. By the end of March 1997, 70,632 pregnant women had been screened. At first, only 11 percent of pregnant women agreed to testing, but the percentage has been in the range of 45 to 55 percent since 1991. The mean (±SD) age of the women was 28±5 years, and the specimens were collected at 12±5 weeks of gestation.
The overall rate of reexamination because of abnormal results was 2.0 percent (1409 of 70,632 women), and 671 women (1.0 percent) were referred for further medical evaluation. Among these 671 women, 171 (25.5 percent) had hyperthyroidism (162 had Graves' disease, and 9 had other causes of hyperthyroidism) and 102 (15.2 percent) had hypothyroidism (67 had chronic autoimmune thyroiditis, and 35 had other causes of hypothyroidism). The overall incidence of the disorders was 1 in 413 and 1 in 692, respectively. In addition, 220 women (32.8 percent) had transient hyperthyroxinemia, and 121 women (18.0 percent) had low serum thyrotropin concentrations. None of the pregnant women with thyroid disease had symptoms or signs of thyroid dysfunction.
We subsequently obtained the results of neonatal screening for congenital hypothyroidism in the infants whose mothers had been referred for medical evaluation during pregnancy. We found six cases of transient hypothyroidism (incidence, 1 in 11,772), eight cases of transient hypothyroxinemia (incidence, 1 in 8829), three cases of neonatal Graves' disease, and one case of transient subclinical hypothyroidism. Thyroid function was not controlled during pregnancy in the mothers of these infants, and all the infants whose mothers' thyroid disorder was well controlled during pregnancy were normal. (The overall incidence of congenital hypothyroidism as detected by neonatal screening is about 1 in 3000 in Sapporo.)
The frequency of thyroid disease among pregnant women may be lower in Japan than elsewhere.1 Nonetheless, we believe that our voluntary program of screening for thyroid disease in early pregnancy is useful because women who have thyroid dysfunction are so often asymptomatic.
Masaru Fukushi, Ph.D.
Kaori Honma, B.S.
Kozo Fujita, M.D., Ph.D.
Sapporo City Institute of Public Health
Sapporo 003-8505, Japan
References
Joseph G. Hollowell, Jr., M.D., M.P.H.
Paul L. Garbe, D.V.M., M.P.H.
Dayton T. Miller, Ph.D.
Centers for Disease Control and Prevention
Atlanta, GA 30341
References
To the Editor: In our study, the seven-to-nine-year-old children of 62 women who had hypothyroidism during pregnancy performed less well on all the administered neuropsychological tests than did the control children. It is highly unlikely that this consistently poorer performance would be due to chance, even though most of the differences in individual test scores were not statistically significant. Performance problems were limited to the 48 children of women whose hypothyroidism was not treated during pregnancy. In this group, the average full-scale IQ score was 7 points lower than that of the control children, 19 percent of the scores were 85 or less (as compared with 5 percent in the control children), and the scores for the majority of tests were significantly poorer. It is this group that deserves the most attention. Herzmann and Torrens suggest several other variables that might influence neuropsychological performance. Table 1 of our study includes an extensive comparison of the women with hypothyroidism and the control women, including an assessment of their socioeconomic status at the time of testing. Given the similarities between the two groups, additional variables are not likely to explain our findings.
Morreale de Escobar and Escobar del Rey raise the point that maternal serum thyroxine concentrations appear to be an important factor in fetal neural development, even in pregnant women with normal serum thyrotropin concentrations. To address their comment, we performed a regression analysis to determine the relation between full-scale IQ scores in the 124 control children and serum free thyroxine values in their mothers. The correlation coefficient was 0.14, with a 10-point gain in IQ predicted from the low end to the high end of the range of serum free thyroxine values (P=0.13). A larger data set would be needed to determine whether this is a chance finding, but the trend in values supports their point.
We thank Fukushi and colleagues for providing the data on prenatal screening for thyroid disease in Sapporo. We agree with Hollowell and colleagues that excessive iodine intake during gestation can have adverse effects on thyroid function.
There was an error in Table 4 of our paper: the first column of P values is for the comparison of the children of the treated women with the children of the control women, not the children of the untreated women.
James E. Haddow, M.D.
Foundation for Blood Research
Scarborough, ME 04074
Robert Z. Klein, M.D.
Dartmouth–Hitchcock Medical Center
Lebanon, NH 03756
Marvin Mitchell, M.D.
University of Massachusetts Medical School
Jamaica Plain, MA 02130
The decrease in median urinary iodine excretion of more than 50 percent in the United States between the period from 1971 to 1974 and the period from 1988 to 1994 that was reported by Hollowell et al.1 was indeed the basis for my recommendation that dietary iodine intake be increased. In their population-based study, not only did 15 percent of women of childbearing age studied in the period from 1988 to 1994 have urinary iodine values of less than 50 µg per liter, but also the median value in these women was 127 µg per liter, and the median value in men of all ages was 160 µg per liter. Urinary iodine values slightly underestimate iodine intake, but given that adults should have an iodine intake of 150 µg daily and pregnant women should have an intake of 200 µg daily,2 it is clear that many people have an inadequate dietary iodine intake.
I agree that "excessive" iodine intake can cause hypothyroidism in patients with certain thyroid diseases, notably chronic autoimmune thyroiditis, but I know of no evidence that an increase in dietary iodine intake of, for example, 300 µg daily can do so.
Given that urinary iodine excretion has fallen substantially in the past 15 to 20 years to values indicative of at least marginally low iodine intake in both women and men, I stand by my conclusion that an increase in iodine intake would benefit everyone, not just pregnant women and their offspring.
Robert D. Utiger, M.D.
References
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