Treatment of Hyperthyroidism in Pregnant Women
Propylthiouracil (PTU) is the drug of choice for treating hyperthyroidism during the first trimester of pregnancy, while methimazole (MMI) is preferred for the second and third trimesters. 1, 2, 3
First-Line Medication Therapy
- PTU should be used during the first trimester due to the possible teratogenicity associated with methimazole exposure during early pregnancy 1, 4
- Consider switching from PTU to methimazole for the second and third trimesters due to the risk of maternal hepatotoxicity with PTU 2, 3
- The goal of treatment is to maintain free T4 or Free Thyroxine Index (FTI) in the high-normal range using the lowest possible thioamide dosage 5, 1
- Monitor thyroid function by measuring free T4 or FTI every 2-4 weeks to adjust dosage appropriately 5, 1
Symptom Management
- Beta-blockers (e.g., propranolol) can be used temporarily to control symptoms until thioamide therapy reduces thyroid hormone levels 5, 1
- In cases of thyroid storm (a rare but serious condition affecting pregnant women with hyperthyroidism), immediate treatment with multiple medications is required, including thioamides, iodide solutions, and dexamethasone 5, 1
Risks and Monitoring
- Untreated hyperthyroidism during pregnancy increases risks of severe preeclampsia, preterm delivery, heart failure, and possibly miscarriage 5, 1
- Monitor for signs of agranulocytosis (sore throat and fever) in patients on thioamides; if present, obtain a complete blood count and discontinue the medication 5, 2
- For women with Graves' disease, monitor for normal heart rate and appropriate fetal growth 1
- Inform the newborn's physician about maternal Graves' disease due to the risk of neonatal thyroid dysfunction 1
Important Considerations
- Radioactive iodine (I-131) treatment is absolutely contraindicated during pregnancy 5, 1
- Thyroidectomy should be reserved only for women who do not respond to thioamide therapy or develop severe adverse reactions to medications 1, 6
- In many pregnant women, thyroid dysfunction diminishes as pregnancy progresses, allowing for dosage reduction or even discontinuation of therapy several weeks before delivery 5, 3
Medication-Specific Considerations
Propylthiouracil (PTU)
- First-line treatment during first trimester 1, 7
- Associated with rare but potentially severe hepatotoxicity 2, 8
- Present in breast milk in clinically insignificant amounts; breastfeeding is generally considered safe 2
Methimazole (MMI)
- Preferred for second and third trimesters 3, 4
- Associated with a specific pattern of rare teratogenic effects after first trimester exposure, including choanal atresia and aplasia cutis congenita 9, 4
- Present in breast milk in small amounts; breastfeeding is generally considered safe 3
Treatment Algorithm
- First trimester: Use PTU at lowest effective dose 1, 7
- Second and third trimesters: Consider switching to MMI 2, 3
- Monitor thyroid function every 2-4 weeks and adjust medication accordingly 5, 1
- Use beta-blockers for symptom control if needed until thioamide therapy takes effect 5, 1
- Consider reducing or discontinuing therapy as pregnancy progresses if thyroid function improves 5, 3
Recent meta-analysis confirms that PTU is associated with lower risk of congenital anomalies compared to MMI, supporting its use in the first trimester 7. However, the benefit of switching between medications during pregnancy remains unclear and requires further study 7.