Thyroid disease is present in 2-5 percent of all women and 1-2 percent of women in the reproductive age group. Thyroid problems are common in women who are pregnant. Several of the thyroid disorders which tend to occur during pregnancy are autoimmune in nature. By this we mean that the body develops antibodies directed against thyroid cells, which then affect the way the thyroid gland functions. Antibodies which damage the thyroid cells may result in lymphocytic thyroiditis (inflammation of the thyroid), also known as Hashimoto’s disease. These damaging antibodies can reduce the function of the thyroid and lead to hypothyroidism. On the other hand, your body can make antibodies against thyroid tissue which can stimulate thyroid cell function. In this case, hyperthyroidism due to over-function of the thyroid (Graves’ disease) may be the result.
Hypothyroidism. If hypothyroidism is suspected in a pregnant patient, the physician can perform a TSH blood test. Just as in non-pregnant women, the TSH will be increased if hypothyroidism is present. If a woman is already being treated with thyroxine when she becomes pregnant, she should continue to take this medication during pregnancy. Thyroxine is safe to take and is well absorbed during pregnancy. Although there is usually no need for a dose change, some women require somewhat higher doses when they are pregnant. Physicians generally monitor the TSH level to detect even mild hypothyroidism and increase the thyroxine dose, if necessary.
Guidelines for treatment
The following is a summary of the key components of the guidelines, which have important implications for women who develop hypothyroidism
- If a woman is diagnosed with hypothyroidism prior to pregnancy, her thyroid medication should be adjusted so that the TSH level is no higher than 2.5 µU/ml prior to becoming pregnant.
- If a woman is diagnosed as hypothyroid during pregnancy, she should be treated without delay, with the goal of restoring her thyroid levels to normal as quickly as possible. During the first trimester, the TSH should be maintained at less than 2.5 µU/ml (and less than 3.0 µU/ml in the second and third trimesters.) After initial diagnosis, thyroid function tests should be reevaluated within 30 to 40 days.
- By the time a woman is four to six weeks pregnant, her dose of thyroid medication will usually need to be increased, potentially by as much as 30 to 50 percent.
- A woman with thyroid autoimmunity (such as she has previously tested positive for thyroid antibodies) who has normal TSH levels in the early stages of herpregnancy is still at risk of becoming hypothyroid at any point in the. She should be monitored regularly through the pregnancy for elevated TSH.
- After childbirth, most women with hypothyroidism will need their dosage of thyroid hormone replacement reduced
Hyperthyroidism. Thyrotoxicosis (hyperthyroidism) during pregnancy, most often due to Graves’ disease, or it can be a transient form that triggers hyperemesis gravidarum –- a condition of pregnancy that causes severe morning sickness.
Guidelines for treatment
The risk of miscarriage and stillbirth is increased if thyrotoxicosis goes untreated, and the overall risks to mother and baby further increase if the disease persists or is first recognized late in pregnancy. The diagnosis is suggested by specific physical signs such as prominent eyes, enlarged thyroid gland, and exaggerated reflexes, and is confirmed by markedly elevated serum thyroid hormone levels.
Radioactive iodine scans or treatment are never performed in pregnancy. However, if a thyroid scan is inadvertently done in pregnancy, this should cause little concern, since the amount of radioactivity delivered to the fetus is barely above the background level in the environment. On the other hand, if radioactive iodine treatment is inadvertently administered in pregnancy, this raises concerns about the radiation effects on the developing fetus in early pregnancy. The amount of radiation may approach levels which can be harmful and, after appropriate counseling, some patients may opt for a therapeutic abortion. Still a number of completely normal infants have been born in this situation. Later in pregnancy radioactive iodine can destroy the fetal thyroid, but this is probably not a sufficient reason to end the pregnancy, since recognition and treatment of hypothyroidism shortly after delivery usually assures normal growth and development in the child.
The treatment of choice for thyrotoxicosis during pregnancy is antithyroid medication, either propylthiouracil or methimazole, since radioactive iodine cannot be used. The initial goal is to control the hyperthyroidism and then use the lowest medication dose possible to maintain the serum thyroid hormone levels in the high normal range. In this way the smaller doses of medications are used, and there seems to be little risk to the baby. If a mild allergy to one of these medications develops, the other medication may be substituted. If there is a problem with taking pills or more severe drug allergy, then an operation may be performed to remove most of the thyroid gland. This is usually done in the middle part of the pregnancy. Fortunately, it is rarely necessary.
The natural course of hyperthyroidism in pregnancy is for the disease to become milder or remit totally near term. In many patients anti-thyroid medications can be tapered to low levels or even discontinued. For those patients who are not so fortunate, it is important to maintain control of the hyperthyroidism throughout pregnancy to avoid severe thyrotoxicosis (thyroid storm) developing during labor and delivery. If this does develop, additional acute treatment with beta-adrenergic blocking drugs are used.
If a woman has a severe negative reaction to anti-thyroid drugs, requires very high doses to control her hyperthyroidism, or has uncontrolled hyperthyroidism despite treatment, surgery may be recommended. The surgery would usually be recommended during the second trimester, when it is least likely to endanger the pregnancy.
In dealing with thyroid disease in pregnancy, the physician and patient should be aware of problems that occur before and after, as well as during the actual pregnancy. There should be equal concern for the welfare of both the mother and baby. Fortunately, most thyroid conditions can be recognized, problems can be anticipated, and effective treatment is available. The outcome is almost always a healthy one, for both the mother and her baby.