How should methimazole (antithyroid medication) be adjusted in a patient undergoing surgery?

Perioperative Management of Methimazole for Surgery

Methimazole should be continued through the perioperative period at the current dose for most patients undergoing elective surgery, with the goal of maintaining euthyroidism to prevent thyroid storm and optimize surgical outcomes. 1

Preoperative Management

Standard Approach for Elective Surgery

• Continue methimazole at the current dose through the day of surgery for patients who are euthyroid or well-controlled on therapy 2
• The primary goal is to achieve and maintain a euthyroid state before any elective surgical procedure, as operating on hyperthyroid patients significantly increases the risk of perioperative complications including thyroid storm 3
• For patients with newly diagnosed or poorly controlled hyperthyroidism, delay elective surgery until euthyroidism is achieved (typically 6-12 weeks of treatment) 4, 3

Timing Considerations

• Do not withhold methimazole on the day of surgery - unlike metformin or other oral hypoglycemics that are held perioperatively, antithyroid medications should be continued to prevent rebound hyperthyroidism 2, 1
• The median preoperative treatment duration to achieve euthyroidism is approximately 12 weeks with methimazole, though this varies based on disease severity, goiter size, and pretreatment thyroid hormone levels 4, 3

Intraoperative Considerations

If Patient Cannot Take Oral Medications

For patients who develop complications preventing oral intake (bowel obstruction, ileus, severe vomiting, or emergent surgery requiring NPO status):

• Intravenous methimazole can be prepared and administered when oral or rectal routes are not feasible 5
• Preparation: 500 mg methimazole USP powder reconstituted with pH-neutral 0.9% sodium chloride to 50 mL final volume, filtered through 0.22-micron filter using aseptic technique 5
• Administration: slow IV push over 2 minutes followed by saline flush 5
• Alternative: Rectal methimazole achieves equivalent blood levels to oral administration (peak levels ~1160-1180 ng/mL) and can be used if IV access is problematic 6

Postoperative Management

Immediate Postoperative Period

• Resume methimazole as soon as oral intake is tolerated, typically within 24 hours of surgery 1
• Monitor for signs of thyroid storm in the first 24-48 hours postoperatively, particularly in patients who were not optimally controlled preoperatively 3
• Clinical signs requiring intervention include: tachycardia out of proportion to surgical stress, fever, altered mental status, or cardiovascular instability 2

Monitoring Requirements

• Check thyroid function tests (TSH, free T4) 2-4 weeks postoperatively to assess for changes in thyroid hormone requirements related to surgical stress 1
• Patients may require dose adjustments based on postoperative thyroid function, particularly if they underwent thyroid or parathyroid surgery 2

Special Circumstances

Emergency Surgery in Uncontrolled Hyperthyroidism

When emergency surgery cannot be delayed for adequate preoperative preparation:

• Administer methimazole at higher doses (40 mg daily or divided doses) combined with supportive measures 4, 7
• Add beta-blockade (metoprolol or propranolol) for symptomatic control and cardiovascular protection 3
• Consider adding inorganic iodine (potassium iodide 38 mg daily) to accelerate achievement of euthyroidism - this combination (methimazole 15 mg + iodine) achieves euthyroidism faster than methimazole 30 mg alone (45% vs 25% within 30 days) 7
• Hydration and supportive care are critical 2

Thyroid Storm Prevention

• The risk of thyroid storm is highest in patients with: severe hyperthyroidism (free T4 ≥5 ng/dL), large goiters, and those inadequately prepared preoperatively 4, 7
• Do not discontinue methimazole perioperatively as this significantly increases thyroid storm risk 2, 1
• If thyroid storm develops postoperatively, treatment includes: propylthiouracil or methimazole, saturated solution of potassium iodide, dexamethasone, beta-blockers, and supportive care 2

Common Pitfalls to Avoid

• Do not treat methimazole like other oral medications that are routinely held perioperatively - continuing antithyroid therapy is essential for patient safety 2, 1
• Do not assume all patients on methimazole are euthyroid - verify thyroid function preoperatively and ensure adequate disease control before elective procedures 4
• Do not forget to communicate with the surgical and anesthesia teams about the patient's thyroid status and medication regimen 1
• Do not overlook drug interactions - methimazole may affect warfarin activity, beta-blocker clearance, and digoxin levels, requiring monitoring and dose adjustments perioperatively 1