Hypothyroidism During Pregnancy Linked To Miscarriage Risk

Hypothyroidism During Pregnancy Linked To Miscarriage RiskSCARBOROUGH, 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 James 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.

(1) Pregnancy; the condition of being pregnant.
(2) The condition of a woman with respect to her having borne viable offspring.
(3) Ability to live after birth; capable of living.
(4) Indicates newborn’s health 1-5 minutes post-birth (color, weight, respiratory rate, and muscle tone).


Hyperthyroidism and Pregnancy

Hyperthyroidism refers to the signs and symptoms which are due to the production of too much thyroid hormone. [ Hyperthyroidism is covered in great deal on other pages on this site (about 8 in all), so only that part of hyperthyroidism which pertains to the pregnant mother will be discussed here]. An overactive thyroid gland (hyperthyroidism) often has its onset in younger women. Because a woman may think that feeling warm, having a hard or fast heartbeats, nervousness, trouble sleeping, or nausea with weight loss are just parts of being pregnant, the symptoms and signs of this condition may be overlooked during pregnancy.

In women who are not pregnant, hyperthyroidism can affect menstrual periods, making them irregular, lighter, or disappear altogether. It may be harder for hyperthyroid women to become pregnant, and they are more likely to have miscarriages. If a woman with infertility or repeated miscarriages has symptoms of hyperthyroidism, it is important to rule out this condition with thyroid blood tests. It is very important that hyperthyroidism be controlled in pregnant women since the risks of miscarriage or birth defects are much higher without therapy. Fortunately, there are effective treatments available. Antithyroid medications cut down the thyroid gland’s overproduction of hormones and are reviewed on another page on this site. When taken faithfully, they control hyperthyroidism within a few weeks. In pregnant women thyroid experts consider propylthiouracil (PTU) the safest drug. Because PTU can also affect the baby’s thyroid gland, it is very important that pregnant women be monitored closely with examinations and blood tests so that the PTU dose can be adjusted. In rare cases when a pregnant woman cannot take PTU for some reason (allergy or other side effects), surgery to remove the thyroid gland is the only alternative and should be undertaken prior to or even during the pregnancy if necessary. Although radioactive iodine is a very effective treatment for other patients with hyperthyroidism, it should never be given during pregnancy because the baby’s thyroid gland could be damaged.

Because treating hyperthyroidism during pregnancy can be a bit tricky, it is usually best for women who plan to have children in the near future to have their thyroid condition permanently cured. Antithyroid medications alone may not be the best approach in these cases because hyperthyroidism often returns when medications is stopped. Radioactive iodine is the most widely recommended permanent treatment with surgical removal being the second (but widely used) choice. It is concentrated by thyroid cells and damages them with little radiation to the rest of the body. This is why it cannot be given to a pregnant woman, since the radioactive iodine could cross the placenta and destroy normal thyroid cells in the baby. The only common side effect of radioactive iodine treatment is underactivity of the thyroid gland, which occurs because too many thyroid cells were destroyed. This can be easily and safely treated with levothyroxine. There is no evidence that radioactive iodine treatment of hyperthyroidism interferes with a woman’s future chances of becoming pregnant and delivering a healthy baby. For more information on the treatment options of hyperthyroidism see our page on this topic.

Thyroid Problems After Pregnancy
¬†One of every twenty women develop thyroid inflammation within a few months after delivery of their baby, a condition called postpartum thyroiditis. This form of thyroid inflammation is painless and causes little or no gland enlargement. However, the condition interferes with the gland’s production of thyroid hormones. Thyroid hormone may leak out of the inflamed gland in large amounts, causing hyperthyroidism that lasts for several weeks. Later on, the injured gland may not be able to make enough thyroid hormone, resulting in temporary hypothyroidism. Symptoms of hyperthyroidism and hypothyroidism may not be recognized when they occur in a new mother. They may be simply attributed to lack of sleep, nervousness, or depression.
Thyroid Symptoms Occasionally Overlooked in New Mothers

Easily upset
Trouble losing weight

Postpartum thyroiditis goes away on its own after one to four months. While it is active, however, women often benefit from treatment for their thyroid hormone excess or deficiency. Some of the symptoms caused by too much thyroid hormone, such as tremor or palpitations, can be improved promptly by medications called beta-blockers(e.g., propranolol). Antithyroid drugs, radioactive iodine, and surgery do not need to be considered because this form of hyperthyroidism is only temporary. If thyroid hormone deficiency develops, it can be treated for one to six months with levothyroxine. Women who have had an episode of postpartum thyroiditis are very likely to develop the problem again after future pregnancies. Although each episode usually resolves completely, one out of four women with postpartum thyroiditis goes on to develop a permanently underactive thyroid gland in future. Of course, levothyroxine fully corrects their thyroid hormone deficiency, and when used in the correct dose, can be safely taken without side effects or complications.

Thyroid Problems in the Baby
¬†Rarely, a baby may be born without a thyroid gland. This birth defect is not caused by thyroid problems in the mother. If an infant’s hypothyroidism is not recognized and treated promptly, he/she will not develop normally. Therefore, all newborn babies in the United States routinely have a blood test to be sure that hypothyroidism is diagnosed and treated. Most thyroid medications will have no effect on the baby. The exception to this generality is the administration of radioactive iodine to the mother during pregnancy. Radioactive iodine can cross the placenta and it can destroy thyroid cells in the fetus.


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