Conversion of Carbimazole 10mg to PTU in Pregnancy
Switch carbimazole 10mg once daily to propylthiouracil (PTU) 150mg once daily during the first trimester, then plan to switch back to carbimazole after week 13-14 to minimize maternal hepatotoxicity risk.
Conversion Ratio and Dosing
- The standard conversion ratio is approximately 1:15 to 1:20 (carbimazole to PTU), meaning carbimazole 10mg daily converts to PTU 150-200mg daily 1, 2.
- Start with PTU 150mg once daily as the most commonly used equivalent dose, which can be adjusted based on thyroid function monitoring 1, 2.
- PTU is the preferred antithyroid medication during the first trimester due to lower risk of congenital abnormalities compared to methimazole/carbimazole 3, 1, 2.
Rationale for Switching
- Carbimazole (which converts to methimazole) is associated with a specific pattern of rare congenital malformations including choanal atresia, aplasia cutis congenita, and other facial, cardiac, gastrointestinal, and skin anomalies when used in the first trimester 4, 5, 6.
- PTU has less clear evidence of teratogenicity in the first trimester, making it the safer choice during organogenesis 4, 6.
- However, PTU carries a risk of severe hepatotoxicity including rare cases requiring liver transplantation, which is why switching back to methimazole/carbimazole after the first trimester is recommended 3, 4, 6.
Treatment Goals and Monitoring
- Maintain free T4 in the high-normal range using the lowest effective PTU dose 3, 1, 2.
- Monitor free T4 or free thyroxine index (FTI) every 2-4 weeks to adjust medication dosage appropriately 1, 2.
- Check TSH level every trimester once thyroid function is stable 1, 2.
Timing of Medication Switch
- Continue PTU through the first trimester (until approximately week 13-14) 3.
- Plan to switch back to methimazole/carbimazole after week 13-14 to minimize maternal hepatotoxicity risk in the second and third trimesters 3.
- The conversion back would be approximately PTU 150mg daily to carbimazole 10mg daily, using the reverse ratio 1, 2.
Critical Safety Warnings
- Advise the patient to immediately seek medical attention if she develops fever, sore throat, malaise, abdominal complaints, or jaundice while on PTU, as these may indicate agranulocytosis or hepatotoxicity 1, 7.
- Untreated or inadequately treated hyperthyroidism poses far greater risks than the medications themselves, including preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 1, 2, 4.
- Never use radioactive iodine during pregnancy as it causes fetal thyroid ablation 1, 2.
Common Pitfall to Avoid
- Do not leave the patient untreated during the medication switch, as inadequately controlled hyperthyroidism is more dangerous than either medication 4, 5.
- Do not switch medications multiple times during the first trimester, as this may result in periods of inadequate thyroid control which itself increases the risk of congenital anomalies 5.