Yes, hyperthyroidism can increase the risk of miscarriage, particularly when it is uncontrolled or untreated during pregnancy. A large Danish population study found that women with hyperthyroidism diagnosed before or during pregnancy had a 28% higher rate of spontaneous abortion compared to the general population. The good news: when thyroid levels are brought under control, the risk drops significantly.
How Excess Thyroid Hormones Disrupt Pregnancy
Thyroid hormones regulate metabolism throughout the body, and during pregnancy they influence blood flow to the placenta, blood sugar levels, and the clotting system. When these hormones are too high, several things go wrong at once. The hyperthyroid state increases insulin resistance and raises blood glucose, both of which can impair early placental development. It also alters coagulation by changing how the body produces clotting proteins, which can either promote abnormal clotting in placental blood vessels or favor bleeding.
Uncontrolled thyrotoxicosis raises the risk of pregnancy-induced high blood pressure, which can reduce the flow of nutrients and oxygen to the developing fetus. In earlier research tracking 60 pregnancies complicated by hyperthyroidism, every case of miscarriage or stillbirth occurred in women whose thyroid levels were either completely untreated or inadequately controlled at the time of delivery. None occurred in women who had been successfully treated to normal thyroid levels.
Graves’ Disease vs. Temporary Pregnancy Thyrotoxicosis
Not all hyperthyroidism in pregnancy carries the same risk. The most common cause of temporarily elevated thyroid hormones during pregnancy is gestational transient thyrotoxicosis, which happens because the pregnancy hormone hCG stimulates the thyroid gland in early weeks. This form typically resolves on its own by the second trimester and is rarely linked to serious complications, though severe cases can cause intense nausea, vomiting, dehydration, and weight loss of 5% or more of pre-pregnancy weight.
Graves’ disease, the main autoimmune cause of hyperthyroidism, is a different story. In Graves’ disease, the immune system produces antibodies called TRAb (thyrotropin receptor antibodies) that overstimulate the thyroid. These antibodies cross the placenta, meaning they can directly affect the fetal thyroid gland. Research published in Clinical Chemistry and Laboratory Medicine found that TRAb is an independent risk factor for pregnancy loss, and that it actually outperforms standard thyroid blood tests as a predictor of miscarriage risk. The optimal cutoff identified in the study had a sensitivity of 83.5% and specificity of 85.3% for assessing the risk of pregnancy loss. When TRAb levels climb above normal, the risk of fetal thyroid problems increases three to five fold.
You can tell the two conditions apart clinically: gestational transient thyrotoxicosis does not involve TRAb antibodies, does not cause eye bulging or a visibly enlarged thyroid, and the thyroid looks normal on ultrasound. Graves’ disease often presents with one or more of those features.
Thyroid Levels During Pregnancy
Pregnancy naturally shifts thyroid function. TSH (the hormone that tells the thyroid how much to produce) runs lower in the first trimester because hCG has a thyroid-stimulating effect. The American Thyroid Association’s 2011 guidelines set trimester-specific ranges: 0.1 to 2.5 mIU/L in the first trimester, 0.2 to 3.0 in the second, and 0.3 to 3.5 in the third. A TSH that falls below these ranges with elevated thyroid hormones points toward hyperthyroidism that may need treatment.
Subclinical hyperthyroidism, where TSH is slightly low but thyroid hormone levels remain in the normal range, is rarely associated with adverse pregnancy outcomes. The pregnancies at highest risk are those with overt, uncontrolled thyrotoxicosis, meaning both a very suppressed TSH and clearly elevated thyroid hormones.
Treatment During Pregnancy
Treating hyperthyroidism in pregnancy requires balancing the risks of uncontrolled disease against the potential side effects of medication. The current approach is to use propylthiouracil (PTU) during the first trimester, then switch to methimazole for the rest of the pregnancy. The reason for the switch: methimazole has been linked to a specific pattern of rare birth defects when used in the first trimester, while PTU carries a small risk of serious liver toxicity with longer use. By splitting the two, doctors minimize both risks.
Even with treatment, the Danish population study found a modest residual increase in miscarriage risk (18% higher than the general population) among women taking antithyroid medication in early pregnancy. This likely reflects the difficulty of achieving perfect thyroid control immediately, rather than a harmful effect of the medication itself, since all the miscarriages and stillbirths in the smaller clinical study occurred in untreated or poorly controlled women.
Planning Pregnancy After Radioactive Iodine
If you have been treated for hyperthyroidism with radioactive iodine (a common definitive treatment for Graves’ disease), current guidelines recommend waiting 6 to 12 months before conceiving. A large real-world study found that pregnancies conceived less than 6 months after treatment had a congenital malformation rate of 13.3%, compared to 7.9% for those conceived between 6 and 11 months. Beyond the 6-month mark, radioactive iodine treatment did not appear to increase the rate of adverse pregnancy outcomes.
After radioactive iodine, most people become hypothyroid (low thyroid) and need replacement hormone for life. Getting the replacement dose dialed in before conception is important, since both too much and too little thyroid hormone affect pregnancy outcomes.
Warning Signs to Watch For
Hyperthyroidism symptoms overlap with normal pregnancy complaints, which makes it easy to miss. A rapid or pounding heartbeat, trembling hands, unexplained weight loss, heat intolerance, and anxiety can all be signs. The clearest red flag is severe, persistent vomiting with weight loss and dehydration (hyperemesis gravidarum), which occurs in roughly 0.3 to 1.0% of pregnancies and is frequently associated with more severe hyperthyroidism.
If you have a history of Graves’ disease, your provider will likely check TRAb levels during pregnancy, especially later in pregnancy when antibody passage across the placenta can directly affect the baby’s thyroid. High TRAb levels in the third trimester are particularly important to monitor because they can cause temporary hyperthyroidism in the newborn.