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Hypothyroidism in Pregnancy and the Potential Effects on the Baby’s Intellectual Development


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Whenever a woman finds out that she is pregnant, she and her partner naturally desire the pregnancy to be a successful one, and for the child to be as healthy as possible. In most cases, everything that can be done to assure a good outcome will be done, including regular doctor visits, good nutrition, avoidance of alcohol, tobacco, and unnecessary medications, and screening for diabetes and other diseases that could affect the baby’s health. Until recently, checking routinely (screening) for possible thyroid problems was not considered important in pregnant women, unless they had the typical symptoms of hypothyroidism or hyperthyroidism.

The reason that doctors haven’t been screening for thyroid problems in pregnant women stems from old research studies that concluded that thyroid hormone from the mother did not cross the placenta in amounts that were large enough to have any effect on the baby. Consequently, it was also thought that whether a woman’s thyroid was functioning normally or not would have little impact on the baby’s development. Studies done in the 1960s did suggest the possibility that women who were hypothyroid might have a higher chance of giving birth to children whose IQ was lower than children of women with normal thyroid function. However, these observations were dismissed by many skeptics, because thyroid function in pregnant women was difficult to measure back in those days, and because of the prevailing thought that thyroid hormone didn’t cross the placenta to any significant degree.

It is now known that some thyroid hormone does cross the placenta. Furthermore, even the small amount of maternal hormone that gets through to the developing fetus is probably important, especially during the first trimester, before the fetus has developed its own thyroid gland. In fact, it is during this crucial 12-week period that the baby’s brain starts to develop.

New study, new results

A 1999 article in the New England Journal of Medicine provided much-needed information on the relationship between the mother’s thyroid function and the baby’s long-term intellectual development. The study was led by Dr. James Haddow and conducted by a group of investigators in Maine, New Hampshire, and Massachusetts.

Women’s blood, which had been drawn and then frozen during a prior pregnancy between 1987 and 1990, was thawed out and analyzed for hypothyroidism by measuring the TSH level in the stored blood sample. The investigators then tracked down 62 women whose thyroid tests showed that they had been hypothyroid during their pregnancy 9-11 years earlier. In most cases, the women had not been aware of any thyroid disease at the time. Using a battery of sophisticated tests, the intellectual development of their children (on an average of 8 years old) was compared to that of 124 children born to mothers whose thyroid function had been normal during the same time period.

The investigators found that, on average, the children born to the hypothyroid mothers performed less well on a variety of tests of intellectual function, and had an average IQ that was 7 points lower than that of the control children. The authors concluded that "systematic screening for hypothyroidism early in pregnancy may be worthwhile…"

Response by medical organizations

Following the publication of this study, many professional organizations provided their views on how the results should be interpreted. One thing on which all groups agreed is the need for any pregnant women found to be hypothyroid to be treated as soon as possible with thyroid hormone.

The American Thyroid Association, The Endocrine Society, and the American College of Obstetricians and Gynecologists called for more research to confirm the observations of Dr. Haddow and his colleagues. Taking issue with the authors of the study, these groups did not recommend screening of all women for hypothyroidism during pregnancy, but suggested that women of childbearing age who are planning a pregnancy and who have a personal or family history of thyroid disease should be screened, since they are at higher risk of having a thyroid problem. These professional societies felt that mass screening for thyroid disease was unwise; first, the study, while provocative, has not been confirmed, and second, although the costs of screening the entire female population are unknown, they are likely to be considerable.

In contrast, the American Association of Clinical Endocrinologists took a more proactive stance, stating that routine screening in early pregnancy was reasonable, and that screening should be done in all women considering pregnancy.

Iodine deficiency?

Similar views were expressed by Dr. Robert Utiger in an editorial in The New England Journal of Medicine that accompanied the paper by Dr. Haddow and his associates. He also raised the idea that some of the hypothyroidism that occurs in pregnancy may be due to iodine deficiency. Although the United States is not considered to be an iodine deficient area of the world, a recent nutritional survey of the population showed that 15% of women of childbearing age were iodine deficient. Dr. Utiger recommended that measures be implemented to guarantee that all people, and especially young women, have an adequate iodine intake. One way to do this would be to ensure that vitamins, especially prenatal vitamins, contain iodine.

What you can do

If you are pregnant or thinking about becoming pregnant, you should discuss the possibility of thyroid testing with your doctor. A family history of thyroid disease, or a personal or family history of other conditions known to be associated with thyroid disease such as prematurely gray hair, vitiligo (white patches on the skin), and the "juvenile" type of diabetes, should prompt testing. Similarly, if you have symptoms of fatigue, heat or cold intolerance, weight loss or gain, menstrual irregularity, problems with energy or sleep, or other thyroid-related symptoms, testing for thyroid disease would be appropriate.

You should also be sure that if you are taking multivitamins or prenatal vitamins, they contain 150 micrograms of iodine in each capsule: this is the recommended daily iodine requirement. Do not overdo it, since too much can be as bad as too little.

It is our hope that your child and all future children will grow up and achieve their full intellectual potential. A better understanding of how thyroid problems affect pregnancy will help to achieve this goal.

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