Is Levothyroxine (Synthroid) Safe with Ketamine?

If you're on Levothyroxine (Synthroid) or another thyroid hormone replacement and considering at-home ketamine therapy, the answer is routine: continue your normal medication on the normal schedule. There's no pharmacologic interaction, no special timing concern, no session-day rule. The one clinically meaningful note on this page is the TSH-screening recommendation at intake, which is good practice for any new depression treatment and not specific to ketamine.

Why this is a short and simple page

Most drug-interaction pages in this directory exist because there's a real clinical conversation to have: a session-day timing rule, a dose-aware nuance, an evidence-based attenuation finding, or an indication-specific subgroup analysis. The Levothyroxine page exists because patients on Synthroid (millions of US adults) reasonably ask the question, but the honest answer is brief: this is a routine medication that doesn't interact with ketamine in any clinically meaningful way.

Thyroid hormone replacement operates on a different biological axis (the hypothalamic-pituitary-thyroid feedback loop) than the neurotransmitter systems ketamine affects. The medications don't share metabolic pathways, don't compete for receptors, and don't have additive cardiovascular or sedation effects in any way that requires accommodation.

The TSH screening note

The most clinically important point on this page isn't about the interaction; it's about making sure your thyroid is actually in target range. Untreated or undertreated hypothyroidism produces symptoms that overlap substantially with depression: fatigue, low energy, cognitive slowing, weight gain, decreased motivation, depressed mood. Patients sometimes arrive on Synthroid that hasn't been adjusted in years, with a TSH that's drifted out of range, and a depression picture that could be partly or wholly attributable to undertreated hypothyroidism.

At intake, we confirm:

A TSH level drawn within the past 12 months. If older, we ask for an updated lab.

Whether the TSH is in target range. For most adults the upper limit of normal is approximately 4.5-5.0 mIU/L; for some specific clinical situations (pregnancy, thyroid cancer history, advanced age) the targets differ.

Whether you have any clinical symptoms suggesting current thyroid dysregulation (cold intolerance, hair changes, dry skin, constipation, weight changes).

If your TSH is above target or symptoms suggest undertreated hypothyroidism, the recommendation is to optimize your Levothyroxine dose with your prescribing physician first. Some patients find that resolving the hypothyroidism alone substantially improves the depression picture, sometimes enough that ketamine therapy isn't needed. Others find that optimizing thyroid helps but the depression component remains and ketamine is still appropriate.

What we do at intake when Levothyroxine is in the medication list

The intake conversation is brief:

The current dose. 25 to 200 mcg/day is standard, with most patients on 50-150 mcg.

When you last had thyroid labs. TSH within 12 months is what we want to see.

Whether you've had recent dose adjustments. Frequent adjustments suggest the optimal dose is still being calibrated.

Any other thyroid-related history (subclinical hypothyroidism, thyroidectomy, Hashimoto's, prior hyperthyroidism treatment, pregnancy on thyroid).

For most patients on stable Levothyroxine with in-range TSH, this is a 2-minute conversation and we proceed with standard ketamine onboarding.

Other thyroid hormone replacements

The same conclusion applies to all forms of thyroid hormone replacement:

Levothyroxine (Synthroid, Levoxyl, Tirosint, Unithroid, Euthyrox) is synthetic T4, the most commonly prescribed form.

Liothyronine (Cytomel) is synthetic T3. Less commonly prescribed than T4; sometimes used as add-on to T4 in patients who don't fully respond to T4 alone.

Natural Desiccated Thyroid (Armour Thyroid, Nature-Throid, NP Thyroid) is dried porcine thyroid gland containing both T3 and T4. Some patients prefer it over synthetic options; the ketamine intake conversation is identical.

T4/T3 combinations are sometimes formulated as custom preparations through compounding pharmacies for patients with specific clinical needs.

All of these continue throughout the ketamine course. None requires any special timing or dose adjustment for ketamine sessions.

Bottom line

Levothyroxine (and other thyroid hormone replacement medications) are routine and unchanged by ketamine therapy. Continue your normal medication on the normal schedule. The only intake step worth mentioning is confirming a recent TSH to make sure your thyroid is in target range and any depression-mimicking symptoms attributable to undertreated hypothyroidism are being addressed. This is good practice for any new depression treatment and not specific to ketamine.