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Hyperthyroidism and Pregnancy

Maternal complications of elevated thyroid function in pregnancy are rare, preventable, and when they develop, treatable. High thyroid function is usually caused by Graves Disease. The body makes its own, extra, thyroid activating hormone, which increases thyroid function. There are two medications commonly used to “turn down” the thyroid.  Both propylthiouracil (PTU) and methimazole suppress maternal thyroid function AND cross the placenta to reach the developing baby.

What to Do

Hyperthyroidism is best controlled before getting pregnant. Super-elevated thyroid function can cause infertility, and problems for the mother and baby. Fetal problems include minor birth defects and early delivery.

A mother’s blood level of thyroid hormone (T4) should be kept “high normal” during pregnancy. High normal is “normal” for pregnancy. T4 is measured frequently, and the medication dose is adjusted accordingly. TSH (thyroid-stimulating hormone test) is not very helpful in managing HYPERthyroid medication.

Most patients can STOP the hyperthyroid medication at 34 weeks. Babies do better if they are born without the medicine “around,” and the rising thyroid function in the mother doesn’t go high enough to cause any problems.


Propylthiouracil, or PTU as it’s referred to, is used most commonly in the United States, and it rarely causes any problems with the baby. Between 1 and 5% of patients can NOT tolerate PTU. They get itching, rash, fever, or liver problems. Sometimes severe problems with the white blood count occur in the mother. Changing to another PTU-like drug helps about half of the time.

Methimazole is an alternative to PTU. We used to think that Methimazole caused problems with the baby’s scalp (fetal aplasia cutis). This is no longer the case. PTU and Methimazole have been compared directly and there was no difference found in fetal effects. The “scalp thing” was proven WRONG.

What is Different

We do EXTRA baby ultrasounds during pregnancy for mom’s taking thyroid-lowering medication. Ultrasound measures baby growth, and can detect heart problems too. Babies with too-little growth OR a slow heartbeat MUST be evaluated for an enlarged thyroid.

A starting dose of PTU is 300 to 450 mg daily. The dose is increased to treat symptoms and to titrate (adjust) the thyroxine levels to “high normal.” A starting dose for Methimazole is 20 – 30 mg, daily.