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