Management of Hyperthyroidism in a Pregnant Woman at 28 Weeks Gestation
Switch to Propylthiouracil (PTU) is the most appropriate management for this pregnant woman in her third trimester with uncontrolled hyperthyroidism on carbimazole. 1, 2
Rationale for Switching to PTU in Third Trimester
- The American Academy of Family Physicians recommends propylthiouracil (PTU) as the preferred antithyroid medication during the first trimester, while methimazole (carbimazole) is preferred in the second and third trimesters 3
- However, when a patient on carbimazole shows signs of uncontrolled hyperthyroidism (symptoms plus low TSH, high T3 & T4), switching to PTU is indicated rather than increasing the carbimazole dose 1
- PTU is considered safer than increasing the dose of carbimazole when faced with breakthrough hyperthyroidism in pregnancy, as higher doses of carbimazole are associated with increased risk of side effects 4
Risks of Inadequate Treatment
- Untreated or inadequately treated hyperthyroidism during pregnancy can lead to serious maternal and fetal complications, including: 2
- Increased risk of maternal heart failure
- Preterm birth
- Stillbirth
- Fetal or neonatal hyperthyroidism
Dosing and Monitoring Recommendations
- When switching to PTU, start with an appropriate dose based on the severity of hyperthyroidism 2
- Monitor free T4 or Free Thyroxine Index (FTI) every 2-4 weeks to adjust dosage appropriately 1
- The goal is to maintain free T4 or FTI in the high-normal range using the lowest possible thioamide dosage 2
- Beta-blockers may be temporarily used to manage symptoms like tremors and palpitations until thyroid hormone levels normalize 2
Important Considerations and Precautions
- While PTU has been associated with hepatotoxicity, this risk must be balanced against the risks of uncontrolled hyperthyroidism in pregnancy 5
- The FDA notes that PTU "may be the treatment of choice when an antithyroid drug is indicated during or just prior to the first trimester of pregnancy" 5, but clinical guidelines support its use throughout pregnancy when indicated 1, 2
- Thyroidectomy should only be considered if the patient does not respond to thioamide therapy or develops severe side effects 2
- Radioactive iodine is absolutely contraindicated during pregnancy 2
Follow-up Care
- Inform the newborn's physician about the maternal hyperthyroidism due to the risk of neonatal thyroid dysfunction 2
- Monitor for signs of agranulocytosis (sore throat and fever) which would require immediate discontinuation of PTU and complete blood count 1
- After delivery, consider switching back to carbimazole/methimazole for long-term management if needed, as PTU has higher risk of hepatotoxicity with prolonged use 5