Treatment of Hyperthyroidism in Pregnant Women
Propylthiouracil (PTU) should be used during the first trimester of pregnancy, then switched to methimazole for the second and third trimesters. 1
First-Line Treatment Approach
First Trimester (Weeks 1-13)
- Use propylthiouracil (PTU) exclusively during the first trimester to avoid methimazole-associated congenital malformations, which include choanal atresia and other rare teratogenic effects. 1, 2
- PTU is specifically recommended by the FDA as the preferred treatment during or just prior to the first trimester of pregnancy, despite its black box warning for hepatotoxicity. 3
- The American College of Obstetricians and Gynecologists explicitly recommends PTU as the preferred antithyroid medication during the first trimester. 1
Second and Third Trimesters (Weeks 14-40)
- Switch from PTU to methimazole after the first trimester to reduce the risk of maternal hepatotoxicity, which can be severe and even fatal with PTU. 1, 4
- This switching strategy balances the teratogenic risk of methimazole in early pregnancy against the hepatotoxic risk of PTU with prolonged use. 2, 5
Treatment Goals and Monitoring
Target Thyroid Function
- Maintain free T4 (FT4) or Free Thyroxine Index (FTI) in the high-normal range using the lowest possible thioamide dosage. 6, 1
- Monitor FT4 or FTI every 2-4 weeks to guide dosage adjustments. 6, 1
- Suppressed TSH with elevated T3 and T4 confirms hyperthyroidism, most commonly from Graves' disease in pregnancy. 1
Symptomatic Management
- Use a beta-blocker (such as propranolol) for symptomatic relief while awaiting thyroid hormone normalization. 6
- Beta-blockers reduce symptoms like tachycardia, tremor, and anxiety during the initial treatment period. 6
Critical Safety Monitoring
Agranulocytosis Warning
- If sore throat and fever develop, obtain a complete blood count immediately and discontinue the thioamide. 6
- Agranulocytosis is a serious side effect that typically presents with these symptoms. 6
Other Serious Side Effects
- Monitor for hepatitis, vasculitis, and thrombocytopenia throughout treatment. 6
- PTU carries a black box warning for severe liver injury and acute liver failure, including cases requiring liver transplantation. 3
Consequences of Untreated Hyperthyroidism
Untreated maternal hyperthyroidism significantly increases risks of:
- Severe preeclampsia 1
- Preterm delivery 1
- Maternal heart failure 1
- Miscarriage 1
- Low birth weight 1
- Fetal or neonatal hyperthyroidism 7
Special Considerations
Fetal Monitoring
- Monitor the fetus for normal heart rate and appropriate growth. 6
- Ultrasound screening for fetal goiter is not necessary unless problems are detected. 6
- Although fetal and neonatal thyroid suppression can occur with thioamide therapy, it is usually transient and rarely requires treatment. 6
Neonatal Communication
- Inform the newborn's physician about maternal Graves' disease due to the risk of neonatal thyroid dysfunction. 6, 1
Breastfeeding
- Women treated with PTU or methimazole can safely breastfeed. 6, 1
- Do not breastfeed for four months after radioactive iodine (I-131) treatment. 6
Contraindications and Alternatives
Radioactive Iodine
- I-131 is absolutely contraindicated in pregnancy. 6
- If inadvertent exposure occurred after 10 weeks of gestation, consider the risk of induced congenital hypothyroidism and whether to continue the pregnancy. 6
Thyroidectomy
- Reserve surgical thyroidectomy for women who do not respond to thioamide therapy or have thioamide intolerance. 6, 8
- If surgery is necessary, perform preferably during the second trimester. 6
Common Pitfalls to Avoid
- Failing to switch from PTU to methimazole after the first trimester increases the risk of maternal hepatotoxicity. 1
- Inadequate monitoring of thyroid function during pregnancy may result in suboptimal treatment. 1
- Using methimazole during the first trimester increases the risk of congenital malformations. 2, 5
- Delaying treatment of clinical hyperthyroidism can lead to serious maternal and fetal complications. 1, 7