Contraindications of Methimazole (MMI)
Methimazole is absolutely contraindicated in patients with hypersensitivity to the drug or any of its components. 1
Primary Contraindications
- Hypersensitivity reactions: Any previous allergic reaction to methimazole or its components represents an absolute contraindication 1
- First trimester of pregnancy: Methimazole is contraindicated during the first trimester due to potential teratogenicity 2
- Breastfeeding mothers: While limited amounts are excreted in breast milk, caution is advised 3
- Severe hepatic impairment: Patients with significant liver disease should avoid methimazole due to risk of hepatotoxicity 4
Relative Contraindications/Use with Caution
Pregnancy Considerations
- First trimester: Propylthiouracil (PTU) is preferred during the first trimester due to methimazole's association with rare congenital anomalies 2, 5
- Second and third trimesters: Methimazole is preferred after the first trimester due to lower risk of hepatotoxicity compared to PTU 2, 5
- Specific birth defects associated with first-trimester use:
- Aplasia cutis (scalp defects)
- Choanal atresia
- Esophageal atresia
- Facial abnormalities 6
Other Precautions
- Agranulocytosis: Although rare, this serious adverse effect requires immediate discontinuation if suspected 7
- Hepatotoxicity: While less common than with PTU, liver function should be monitored 4
- Renal impairment: Dose adjustment may be needed in patients with significant kidney disease
Clinical Decision Algorithm
For non-pregnant patients with hyperthyroidism:
- Methimazole is first-line therapy unless contraindicated by hypersensitivity
- Monitor for side effects including rash, hepatotoxicity, and agranulocytosis
For pregnant patients with hyperthyroidism:
- First trimester: Use PTU (avoid methimazole)
- Second and third trimesters: Switch to methimazole due to lower hepatotoxicity risk 2
- Monitor thyroid function closely throughout pregnancy
For patients with hypersensitivity to methimazole:
- Switch to PTU as alternative therapy
- Consider desensitization to methimazole under allergist supervision if PTU is not tolerated 7
- Plan for definitive therapy (radioactive iodine after pregnancy or surgery) if appropriate
Important Clinical Considerations
- Patients should be educated about signs of agranulocytosis (fever, sore throat) and hepatotoxicity (jaundice, abdominal pain)
- Regular monitoring of complete blood count and liver function tests is recommended
- For patients with hypersensitivity reactions that are not severe (e.g., mild rash), desensitization protocols under allergist supervision may be considered 7
- Uncontrolled hyperthyroidism during pregnancy poses greater risks than controlled medication use, including increased risk of preterm labor and low birth weight 6
Remember that the benefits of treating hyperthyroidism generally outweigh the risks of medication, but proper selection of therapy based on individual patient factors is essential for optimal outcomes.