For the most part, the causes of thyroid disease during pregnancy are the same as in the general population. And the most common thyroid diseases in pregnancy mirror the most common thyroid diseases in the general population: Hashimoto’s disease is the most common thyroid disease in pregnancy, followed by Graves’ disease. In both cases, the risk spikes during the ?rst three months of pregnancy, and then spikes again in the ?rst six months after delivery. Up to 20 percent of all women, particularly those with thyroid antibodies or insulin-dependent diabetes, will develop postpartum thyroiditis. This usually resolves on its own, but 25 percent of the time, it can leave women permanently hypothyroid.
Just as in the general population, pregnant women can develop hypothyroidism or thyrotoxicosis for other reasons. What seems clear to thyroid researchers, however, is that pregnancy increases thyroid hormone requirements; increases the risks of iodine de?ciency, which can increase the severity of preexisting hypothyroidism; can worsen preexisting Hashimoto’s or Graves’ disease; and can unveil overt hypothyroidism in women who had subclinical hypothyroidism prior to pregnancy.
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If hypothyroidism is suspected while you’re pregnant, your doctor will give you a TSH test. Just as in nonpregnant women, your TSH levels will be increased if you’re hypothyroid and you’ll be treated with thyroid hormone replacement. Sometimes pregnancy itself can mask hypothyroid symptoms. For example, constipation, puf? ness, and fatigue are all traits of pregnancy as well as of hypothyroidism.
These symptoms will likely persist after delivery if your hypothyroidism remains untreated, and they can cause serious pregnancy complications and interfere with your postpartum health.
Gestational hypertension, preeclampsia, and eclampsia are more common in women with overt or subclinical hypothyroidism. These pregnancy complications may warrant early delivery or lead to premature delivery.
Gestational thyrotoxicosis refers to a transient form of thyrotoxicosis caused by rising levels of human chorionic gonadotropin (HCG), which stimulate the thyroid gland to make thyroid hormone. This is usually found in women with severe morning sickness and is typically diagnosed toward the end of the ? rst trimester. Gestational thyrotoxicosis usually resolves on its own after pregnancy. Temporary treatment with propranolol, a beta-blocker, may be used.
However, the necessity of the beta-blocker and the length of time you’re on one during pregnancy needs to be carefully monitored on a case-by-case basis by your doctor. If the thyrotoxicosis is very severe, antithyroid drugs, such as propylthiouracil (PTU), may be used in smaller than usual dosages.
Thyrotoxicosis Due to Molar Pregnancy
Roughly one in every ? fteen hundred to two thousand pregnancies in North America will develop into a molar pregnancy, also known as a hydatidiform mole, a form of gestational trophoblastic neoplasia. Here the placenta, in a cruel and bizarre twist of biology, becomes precancerous. This condition is most frequently reported in Asian women and in women in the South Paci? c and Mexico. One sign of a molar pregnancy is thyrotoxicosis, due to very high levels of HCG, which drop only slightly over time after the pregnancy ends.
This is very rare, but molar pregnancy should be ruled out if you become thyrotoxic.
Diagnosis and treatment of hyperthyroidism during pregnancy presents some unique fetal and maternal considerations. First, the risk of miscarriage and stillbirth is increased if hyperthyroidism goes untreated. Second, the overall risks to you and the baby increase if the disease persists or is ?rst recognized late in pregnancy. As in nonpregnant women, speci? c hyperthyroid symptoms usually indicate a problem, but here again, some of the classic symptoms, such as heat intolerance or palpitations, can mirror classic pregnancy complaints.
Usually, symptoms such as bulgy eyes or a pronounced goiter give Graves’ disease away. But because radioactive iodine scans or treatment are never performed during pregnancy, gestational hyperthyroidism can only be con? rmed through a blood test. (If you are exposed to radioactive iodine during pregnancy because the pregnancy was not suspected, you may want to discuss the risks and the possibility of a therapeutic abortion with your practitioner.) If you are of an age to be fertile, you should get a pregnancy test before receiving any radioactive substance, either for scans or for treatments.
The treatment for hyperthyroidism in pregnancy is antithyroid medication. Propylthiouracil (PTU) or methimazole are most commonly used, but PTU is the one usually used during pregnancy. PTU, by suppressing the fetal thyroid, actually bene?ts the fetus. Since the fetal thyroid is slightly more sensitive to PTU than the mother’s thyroid, the dose is slightly less than would completely normalize the mother’s thyroid hormone levels.
Sometimes women discover they are allergic to PTU. If this happens, methimazole is used instead. When there is a problem with both drugs, sometimes a thyroidectomy during the second trimester is performed, although this is rare. In general, surgery is avoided during pregnancy because it can trigger a miscarriage.
Many times, hyperthyroidism becomes milder as the pregnancy progresses. When this happens, antithyroid medication can be tapered off slowly as the pregnancy reaches full term; often, normal thyroid function resumes after delivery. Careful thought should be given to breastfeeding, since radioactive iodine treatments should not be given while breastfeeding nor for at least one month after weaning.
Sometimes beta-blockers such as propranolol are given in addition to PTU. However, this is something your physician must assess on a case-by-case basis because it is not usual care. It can be continued safely during pregnancy if absolutely necessary. One potential risk of using it for too long is having a smaller than average baby.
The Risk of Miscarriage
Studies indicate that women with antithyroid antibodies or with subclinical Hashimoto’s disease or Graves’ disease have a 32 percent risk of miscarriage, compared to a 16 percent risk in women without these conditions. The risk of miscarriage also rises due to age. In the general population of healthy pregnant women under age thirty five, one in six pregnancies ends in miscarriage, with risk at its highest point during the ?rst trimester.