Methimazole is the Drug of Choice for Hyperthyroidism in the Second Trimester of Pregnancy
Methimazole (Tapazole) is the preferred medication for treating hyperthyroidism during the second trimester of pregnancy due to its lower risk of hepatotoxicity compared to propylthiouracil (PTU). 1
Rationale for Medication Selection by Trimester
First Trimester
- Propylthiouracil (PTU) is preferred in the first trimester due to potential teratogenicity associated with methimazole 1
- Methimazole has been associated with a pattern of congenital anomalies including choanal atresia and aplasia cutis congenita when used in early pregnancy 2
Second and Third Trimesters
- Methimazole is preferred in the second and third trimesters 1
- PTU is associated with a higher risk of severe hepatotoxicity, making it less suitable for long-term use beyond the first trimester 3
- The evidence strongly supports using PTU in the first trimester followed by methimazole for the remainder of pregnancy as the safest approach 3
Dosing and Administration
- Initial dosing should aim to maintain free T4 or Free T4 Index in the high-normal range using the lowest possible dose 1
- Methimazole has a longer half-life than PTU, allowing for once-daily dosing 3
- Typical maintenance doses range from 5-15 mg daily 3
- Monitor thyroid function every 2-4 weeks initially until stable 1, 3
- Take with food to minimize gastrointestinal side effects 3
Monitoring During Treatment
- Measure free T4 or FTI every two to four weeks to adjust dosing appropriately 1
- Monitor for signs of agranulocytosis (sore throat, fever) which requires immediate discontinuation and CBC testing 1
- Watch for symptoms of hepatitis, vasculitis, and thrombocytopenia 1
- Beta blockers (e.g., propranolol) can be used temporarily to control symptoms until methimazole reduces thyroid hormone levels 1
Safety Considerations
- Both methimazole and PTU cross the placenta and can affect fetal thyroid function 4
- Suppression of fetal and neonatal thyroid function with thioamide therapy is usually transient 1
- Women treated with methimazole can safely breastfeed 1
- The newborn's physician should be informed of maternal hyperthyroidism treatment 1
Important Cautions
- Untreated hyperthyroidism poses greater risks to both mother and fetus than properly managed medication therapy 5
- Inadequately treated hyperthyroidism increases risk of maternal heart failure, spontaneous abortion, preterm birth, and stillbirth 4
- Thyroid function often improves as pregnancy progresses, potentially allowing for dose reduction later in pregnancy 1
- Radioactive iodine (I-131) treatment is absolutely contraindicated during pregnancy 1
By following this evidence-based approach of using methimazole in the second trimester, clinicians can effectively manage maternal hyperthyroidism while minimizing risks to both mother and fetus.