Pregnancy and Hyperthyroidism: Safe Medication Options and Treatment Guidelines

Women with hyperthyroidism are generally advised to achieve stable thyroid control before attempting pregnancy. Unmanaged overactive thyroid function can lead to complications such as preeclampsia, preterm birth, low birth weight, and even fetal developmental issues. Therefore, proper planning and medical supervision are essential for women who have a history of hyperthyroidism and are considering motherhood.

Understanding Hyperthyroidism During Pregnancy

Hyperthyroidism occurs when the thyroid gland produces excessive amounts of thyroid hormones, leading to symptoms like rapid heartbeat, weight loss, anxiety, and heat intolerance. During pregnancy, this condition requires careful management because both under-treatment and over-treatment can pose risks to the mother and developing baby.

Common Treatment Approaches for Hyperthyroidism

There are three primary treatment options for hyperthyroidism: antithyroid medications, radioactive iodine (iodine-131), and surgical thyroidectomy. However, during pregnancy, not all methods are safe. Radioactive iodine is strictly contraindicated throughout gestation because it can cross the placenta and destroy the fetal thyroid gland, potentially causing permanent hypothyroidism in the unborn child.

Surgical intervention, such as partial or total thyroidectomy, may be considered if medication is ineffective or not tolerated. However, surgery carries increased risks during the first and third trimesters due to higher chances of miscarriage and preterm labor. The second trimester—specifically between weeks 13 and 26—is considered the safest window for surgical management if absolutely necessary.

Antithyroid Drugs: The Preferred Option in Pregnancy

For most pregnant patients, pharmacological treatment remains the safest and most effective approach. The goal is to maintain maternal thyroid function within the upper-normal range using the lowest effective dose of medication, minimizing potential side effects on the fetus.

Choosing Between PTU and Methimazole

Currently, propylthiouracil (PTU) is the preferred antithyroid drug during early pregnancy—especially in the first trimester. This is because PTU binds more strongly to plasma proteins, resulting in lower transfer across the placenta and reduced exposure to the developing fetus. Additionally, studies suggest a lower risk of congenital malformations with PTU compared to methimazole during organogenesis (weeks 5–10 of fetal development).

After the first trimester, some clinicians may switch from PTU to methimazole (MMI), particularly if liver function tests remain stable. While methimazole is associated with a slightly higher risk of teratogenic effects early on, it has a longer half-life and fewer daily dosing requirements, making it more convenient for long-term use in the second and third trimesters. Any transition between medications should be closely monitored by an endocrinologist and obstetrician team.

Balancing Maternal and Fetal Health

Close monitoring through regular blood tests—including TSH, free T4, and sometimes TRAb levels—is crucial throughout pregnancy. Adjustments in dosage may be needed as thyroid hormone demands fluctuate during different stages of gestation. Over-treatment that leads to hypothyroidism can impair fetal brain development, while under-treatment increases the risk of maternal complications.

If surgery becomes necessary—for example, in cases of large goiters causing compression or severe side effects from medication—it should ideally be scheduled during mid-pregnancy. A multidisciplinary team including maternal-fetal medicine specialists, endocrinologists, and surgeons should collaborate to ensure optimal outcomes.

Important Considerations for Postpartum and Breastfeeding

Mothers treated with antithyroid drugs should know that both PTU and methimazole pass into breast milk in small amounts. However, current guidelines consider both drugs acceptable during lactation when used at standard doses, with PTU often preferred due to its lower excretion in milk. Infants should be monitored for signs of hypothyroidism or liver dysfunction, though adverse effects are rare.

In summary, managing hyperthyroidism during pregnancy requires a personalized, cautious, and well-coordinated approach. With appropriate medication selection—primarily PTU in early pregnancy—and timely medical interventions, most women can expect healthy pregnancies and positive outcomes for both themselves and their babies.