RESEARCH - Hypothyroidism during pregnancy linked to miscarriage risk

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Hypothyroidism During Pregnancy Linked To Miscarriage Risk

SCARBOROUGH, ME -- November 22, 2000 -- Pregnant women with

hypothyroidism (underactive thyroid) have a four-times greater risk for

miscarriage during the second trimester, according to a study published in

the Journal of Medical Screening, a specialty publication of the British

Medical Journal.

This is the first large, population-based study to examine pregnancy

complications among women with elevated TSH (thyroid stimulating hormone)

values. The findings indicate that women with hypothyroidism during

pregnancy have a 3.8 percent risk for late miscarriage as opposed to women

with normal thyroid function who only have a 0.9 percent rate. In this

study, 6 out of every 100 late miscarriages could be attributed to thyroid

deficiency during pregnancy, according to the researchers at the Foundation

for Blood Research (FBR).

In a study published in the August 18, 1999 New England Journal of

Medicine, the same researchers documented an association between undetected

subclinical hypothyroidism during pregnancy and lower I.Q. in offspring.

Women with untreated thyroid deficiency during pregnancy are four times more

likely to have children with lower I.Q. scores. Nineteen percent of the

children whose mothers had undiagnosed hypothyroidism during pregnancy

averaged 85 or less on their I.Q. tests. Children who have an I.Q. less than

85 are more likely to have difficulties in school, and may be less

successful in their careers and interpersonal relationships.

" Our current study indicates that a change in pregnancy screening

practices may be warranted, " said Dr. Walter Allan, M.D., lead study author

and director of clinical services at the Foundation for Blood Research.

" Perhaps expectant mothers should get a TSH test before pregnancy or as part

of the initial standard prenatal blood work. "

Other studies among pregnant women with hypothyroidism have suggested

a connection between miscarriage, premature birth, low birthweight,

placental abruption and pregnancy-induced hypertension, however these

studies were limited to women attending high-risk or specialty clinics and

might not have reflected the findings in the general population.

Hypothyroidism is a deficiency in the thyroid, a butterfly-shaped

gland just below the Adam's apple, that plays a critical role in regulating

the most important functions in the body including heart rate, metabolism,

growth, cognitive function and development, energy and mood. Approximately

one out of every 50 women in the U.S. is thyroid deficient during pregnancy.

However, this condition does not only strike during pregnancy. In fact,

nearly 27 million Americans have a thyroid disorder, yet more than half

remain undiagnosed. The condition becomes even more prevalent as women age;

by age 60, one in five women will suffer from a thyroid deficiency.

Thyroid disease can be diagnosed through a simple blood test called

TSH (thyroid-stimulating hormone). This highly sensitive test enables

doctors to detect thyroid disorders early, and in many cases before the

woman experiences symptoms. If left untreated, thyroid disease can lead to

serious long-term complications such as heart disease, osteoporosis,

infertility, impaired I.Q. in offspring, and now potentially, late

miscarriage.

Among the 9,403 women with singleton pregnancies TSH levels were 6mU/L

or greater in 209 (2.2 percent) cases. The rate of late fetal death

(miscarriage) was significantly higher in those pregnancies (8 out of 209 or

3.8 percent) than in women with TSH less than 6 mU/L (83 out of 9,194 or 0.9

percent). Furthermore, the rate of fetal death increased incrementally as

TSH levels increased. Among the 37 women with TSH levels greater than

10mU/L, fetal deaths occurred in 8.1 percent. In the study, six out of every

100 miscarriages could be attributed to thyroid deficiency during pregnancy.

" Little is known about the cause of late miscarriages, but our

findings offer a new opportunity to possibly prevent some of these, " said

Haddow, medical director, Foundation for Blood Research. " Further

research may show that early detection and treatment for maternal

hypothyroidism is the key to preventing these miscarriages. "

The purpose of the study was to examine the relationship between

certain pregnancy complications and TSH levels in pregnant women. Between

July 1990 and June 1992, approximately 10,500 women from the state of Maine

agreed to participate in a study of hypothyroidism, during routine testing

between 15 and 18 weeks' gestation to detect neural tube defects and Down

syndrome. From this pool, it was determined that 9,403 women were eligible

for the study and underwent TSH testing.

The women provided selected information about their pregnancy (e.g.

gravidity(1), parity(2), vaginal bleeding, and smoking status) at the time

of enrollment. Information about pregnancy outcome (e.g. viability(3),

length of gestation, birth weight and Apgar(4) score) was obtained via a

collaborative agreement with the state's Bureau of Vital Records. The serum

TSH measurements were performed at the New England Newborn Screening Program

in Boston and additional thyroid function testing was performed on all serum

samples with TSH levels at or above 6mU/L (the definition of thyroid

deficiency for the current study) at Beth Israel Deaconess Medical Center.

Thyroid function testing was also performed in a selected subgroup of

controls.

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