How many days before attempting pregnancy should I stop carbimazole?

Carbimazole Discontinuation Before Pregnancy

Stop carbimazole immediately when planning pregnancy and switch to propylthiouracil (PTU) for the first trimester, then consider switching back to low-dose carbimazole for the second and third trimesters. 1

Timing and Drug Selection Strategy

Pre-Conception Period

• Do not continue carbimazole when attempting to conceive—switch to PTU before conception attempts begin, as carbimazole is associated with congenital malformations when exposure occurs during the first trimester. 1, 2
• PTU is the recommended antithyroid drug during the first trimester of pregnancy because it has not been associated with increased risk of congenital malformations. 1

Specific Malformation Risks with Carbimazole

• First-trimester carbimazole exposure is linked to a pattern of birth defects including:
  • Aplasia cutis (scalp defects) 2, 3
  • Choanal atresia (blocked nasal passages) 1, 2, 3
  • Esophageal atresia and tracheo-esophageal fistula 1, 2
  • Omphalocele and abdominal wall defects 1, 3
  • Patent vitellointestinal duct 2, 3
• The absolute risk appears very small, but the pattern of defects is consistent enough across multiple case series to warrant avoidance. 1, 2, 3

Algorithmic Approach to Medication Management

Step 1: Pre-Conception Counseling

• All women of reproductive age taking carbimazole should receive preconception counseling about medication risks and the need to switch therapy before attempting pregnancy. 4
• Ensure reliable contraception is in place until the medication switch is complete and thyroid function is stable on PTU. 5

Step 2: Medication Transition Timeline

• Switch from carbimazole to PTU before discontinuing contraception and before actively trying to conceive. 1
• Allow time to achieve stable thyroid control on PTU before conception (typically 4-6 weeks to assess response). 1
• There is no specific "washout period" required for carbimazole itself—the key is ensuring PTU coverage is established before first-trimester exposure occurs. 1

Step 3: Trimester-Specific Management

• First trimester (weeks 0-13): Continue PTU as the preferred agent. 1
• Second and third trimesters (weeks 14-40): Consider switching back to low-dose carbimazole/methimazole, as PTU carries a risk of severe hepatotoxicity (approximately 0.1% of adults) that may outweigh the teratogenic risk after organogenesis is complete. 1, 6

Critical Clinical Pitfalls to Avoid

Hepatotoxicity Risk with PTU

• PTU can cause severe, potentially fatal hepatic failure, particularly in children but also reported in adults. 6
• The risk-benefit calculation favors PTU only during the first trimester when carbimazole's teratogenic risk is highest. 1, 6
• After the first trimester, switching back to carbimazole reduces the cumulative hepatotoxicity risk from prolonged PTU exposure. 1

If Pregnancy Occurs While on Carbimazole

• Do not panic—the absolute risk of malformations remains relatively small even with first-trimester exposure. 1
• Switch immediately to PTU and provide reassurance that most pregnancies exposed to carbimazole result in normal outcomes. 1
• Refer for high-risk obstetric consultation for detailed fetal anatomic surveillance, particularly targeting the specific malformations associated with carbimazole. 4, 2, 3

Disease Control is Essential

• Uncontrolled maternal hyperthyroidism itself increases risks of maternal, fetal, and neonatal complications. 1
• Never discontinue antithyroid medication entirely—always maintain thyroid control with an appropriate agent throughout pregnancy. 1
• The goal is to use the lowest effective dose of antithyroid medication to maintain maternal euthyroidism. 1

Comparison to Other Teratogenic Medications

While the evidence provided includes guidance on other medications, carbimazole differs importantly:

• Methotrexate requires 3 months pre-conception discontinuation due to tissue accumulation and complete contraindication in pregnancy. 4, 7
• Mycophenolate requires 6 weeks pre-conception discontinuation due to high miscarriage and malformation risk. 4
• Carbimazole requires immediate switch to PTU rather than complete discontinuation, because maternal hyperthyroidism must remain controlled throughout pregnancy. 1

Monitoring and Follow-Up

• Ensure thyroid function tests are optimized before conception to minimize the dose of antithyroid medication needed during pregnancy. 4
• Continue close monitoring of thyroid function throughout pregnancy, as medication requirements typically decrease as pregnancy progresses. 1
• Coordinate care between endocrinology and obstetrics, particularly if switching medications mid-pregnancy. 4