Methimazole Administration During Active Labor
Do not initiate methimazole during active labor in a hemodynamically stable pregnant woman with controlled hyperthyroidism; proceed with delivery without delay. 1
Clinical Decision Framework
When Methimazole Should NOT Be Given in Labor
In the absence of thyroid storm, antithyroid medication should not be started during active labor. 1 The rationale is straightforward:
- Antithyroid drugs require days to weeks to achieve therapeutic reduction in circulating thyroid hormones, making initiation during labor physiologically futile 1
- If the patient is hemodynamically stable (no fever, normal heart rate, no altered mental status, no tremor, stable blood pressure) and biochemically controlled (suppressed TSH but free T3 and free T4 within reference ranges), delivery should proceed immediately 1
The Critical Exception: Thyroid Storm
Thyroid storm is the only scenario requiring immediate methimazole administration during labor. 1, 2 This medical emergency demands treatment without waiting for laboratory confirmation and includes:
Emergency treatment protocol:
- Propylthiouracil or methimazole 1, 2
- Saturated potassium iodide or sodium iodide solutions 1, 2
- Dexamethasone 1, 2
- Phenobarbital 1, 2
- Supportive measures (oxygen, antipyretics, hemodynamic monitoring) 1
Clinical presentation of thyroid storm:
- Fever with disproportionate tachycardia 1, 2
- Altered mental status (nervousness, restlessness, confusion, seizures) 1
- Gastrointestinal symptoms (vomiting, diarrhea) 1, 2
- Cardiac arrhythmias 1
Delivery should be avoided during active thyroid storm unless absolutely necessary, as the crisis significantly worsens both maternal and fetal outcomes 1, 2
Post-Delivery Management
After delivery, reassess thyroid function within 1–2 weeks; if abnormal, methimazole may be restarted at that time. 1 This timing allows for:
- Appropriate evaluation of post-partum thyroid status 1
- Safe initiation of therapy when the drug can achieve therapeutic effect 1
- Both methimazole and propylthiouracil are compatible with breastfeeding 1, 2, 3
Safety Considerations for Methimazole Use
First-Trimester Contraindication
Methimazole is contraindicated in the first trimester due to rare but serious congenital malformations, including aplasia cutis, choanal atresia, esophageal atresia with or without tracheoesophageal fistula, and omphalocele 3, 4, 5, 6
Recommended Pregnancy Approach
Propylthiouracil should be used exclusively during the first trimester, then switched to methimazole for the second and third trimesters to minimize both congenital malformations and maternal hepatotoxicity 1, 2, 4, 5
Critical Adverse Effects to Monitor
- Agranulocytosis (sore throat, fever) requires immediate complete blood count and drug discontinuation if confirmed 1, 3
- Hepatotoxicity is less common with methimazole than propylthiouracil but still requires vigilance 3
- Vasculitis (including ANCA-positive vasculitis) can cause severe complications including acute kidney injury, pulmonary hemorrhage, and CNS involvement 3
Common Pitfall to Avoid
Do not confuse routine antepartum management with intrapartum management. While methimazole is appropriate for second and third trimester maintenance therapy 1, 2, initiating it during active labor serves no therapeutic purpose and only delays delivery in a stable patient 1