Ketamine and Levothyroxine Interaction
Concurrent use of ketamine and levothyroxine can produce marked hypertension and tachycardia, requiring close monitoring of blood pressure and heart rate. 1
Direct Drug Interaction
The FDA levothyroxine drug label explicitly warns about this interaction:
- Ketamine combined with levothyroxine may cause marked hypertension and tachycardia 1
- Blood pressure and heart rate must be closely monitored when these drugs are used together 1
- This is a pharmacodynamic interaction (additive cardiovascular effects) rather than an absorption-based interaction 1
Mechanism of Interaction
The interaction occurs through additive cardiovascular stimulation:
- Ketamine produces dose-dependent increases in heart rate, blood pressure, and cardiac output through sympathetic nervous system stimulation 2, 3
- Levothyroxine increases metabolic rate and cardiovascular responsiveness 1
- When combined, these effects are amplified, creating potentially dangerous hemodynamic changes 1
Clinical Implications
This is NOT an absorption interaction - ketamine does not affect levothyroxine absorption from the gastrointestinal tract. The concern is entirely cardiovascular:
- The interaction is most dangerous in patients with pre-existing ischemic heart disease, cerebrovascular disease, or hypertension 2, 3
- Ketamine should be avoided or used with extreme caution in these cardiovascular conditions 2
- Emergence reactions (hallucinations, delirium) occur in 10-30% of adults receiving ketamine and can be minimized with midazolam co-administration 2, 3
Monitoring Requirements
If ketamine must be used in a patient taking levothyroxine:
- Continuously monitor blood pressure and heart rate throughout ketamine administration 1
- Have antihypertensive medications readily available 1
- Consider reducing ketamine dose or using alternative sedation agents in high-risk patients 2
- Ensure thyroid function is optimized (euthyroid state) before elective procedures requiring ketamine 4
Drugs That Actually Affect Levothyroxine Absorption
For context, medications that genuinely impair levothyroxine absorption include:
- Sevelamer hydrochloride and chromium picolinate significantly decrease levothyroxine absorption 5
- Proton-pump inhibitors reduce levothyroxine bioavailability 6
- Soybean flour, cottonseed meal, walnuts, and dietary fiber bind levothyroxine in the GI tract 1
- These should be separated from levothyroxine by several hours 5
Ketamine is not among the drugs that affect levothyroxine absorption - the concern is purely hemodynamic instability from combined cardiovascular stimulation. 1