Perioperative Management of Levothyroxine (Synthroid)
Levothyroxine has a long half-life of 7 days and does not need to be administered intra-operatively if the morning dose is missed on the day of surgery.
Pharmacokinetics of Levothyroxine
- Levothyroxine (Synthroid) has a long half-life of approximately 7 days, which provides a significant buffer against missed doses 1
- The long half-life means that occasional missing of a tablet causes no harm to the patient's thyroid status 1
- Once-daily administration is sufficient due to this extended half-life 1
- The extrathyroidal conversion of T4 into T3 remains fully operative even with occasional missed doses 1
Perioperative Management Recommendations
- For patients on chronic levothyroxine therapy, a single missed dose on the day of surgery does not require intra-operative administration 1, 2
- The stable serum levels of thyroid hormone due to the long half-life prevent acute hypothyroidism from developing with a single missed dose 1, 3
- Patients can safely resume their regular levothyroxine dosing schedule the day after surgery 2
- Unlike medications with short half-lives (such as heparin products that require specific timing of pre-operative discontinuation), levothyroxine's long half-life provides protection against acute changes in thyroid status 1, 2
Special Considerations
- For patients undergoing thyroidectomy who are starting levothyroxine therapy post-operatively, dosing should begin 5 days after surgery 4
- Initial post-thyroidectomy dosing is typically weight-based at approximately 1.6 μg/kg daily 3
- Approximately 75% of post-thyroidectomy patients require dose adjustments after initial dosing, suggesting factors beyond body weight affect appropriate dosing 3
- Thyroid function testing should be performed approximately 6 weeks after surgery to assess the adequacy of replacement therapy 4
Potential Pitfalls
- Avoid confusing levothyroxine with medications that have short half-lives requiring strict perioperative management protocols 1, 2
- Unlike anticoagulants (such as heparin or warfarin) that require careful perioperative bridging protocols, levothyroxine does not require special perioperative dosing considerations for a single missed dose 1
- For patients with severe hypothyroidism who have been recently started on levothyroxine therapy, consultation with endocrinology may be warranted, but this is not necessary for stable patients on chronic therapy 2
- A new equilibrium after dose adjustments takes approximately 6 weeks, so any changes to dosing should not be evaluated earlier than this timeframe 1