Drug & supplement interaction

L-carnitine + Levothyroxine Sodium

Levothyroxine Sodium brand names: Tirosint-Sol, Thyquidity, Ermez.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

L-carnitine has a moderate interaction with Levothyroxine Sodium. Theoretically, L-carnitine might decrease the effectiveness of thyroid hormone replacement.

Read the full interaction report
SeverityModerate
LikelihoodProbable
EvidenceB
What do Severity, Likelihood & Evidence mean?
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Interaction report

L-carnitine and Levothyroxine Sodium

Here is the interaction we found between these two, with the evidence behind it.

Moderate

THYROID HORMONE

Why this applies to Levothyroxine Sodium

Levothyroxine Sodium falls under THYROID HORMONE — the group L-carnitine interacts with.

Theoretically, L-carnitine might decrease the effectiveness of thyroid hormone replacement.
L-carnitine appears to act as a peripheral thyroid hormone antagonist by inhibiting entry of thyroid hormone into the nucleus of cells. Taking L-carnitine also seems to diminish some of the symptoms of hyperthyroidism.
Severity Moderate Likelihood Probable Evidence B

Interaction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports..

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the gist with levothyroxine (your thyroid replacement) and L-carnitine. Levothyroxine isn't a prodrug, it just replaces the thyroid hormone your body needs. The concern is that L-carnitine seems to act as a peripheral blocker, getting in the way of thyroid hormone entering the inside of your cells where it does its work. So in theory, regular L-carnitine could chip away at how well your levothyroxine is working.

If that happened, you might notice hypothyroid symptoms creeping back, things like fatigue, feeling cold, weight gain, or sluggishness. To be fair, this is based on limited, lower-quality evidence, so it's more of a heads-up than an alarm. If you take both, mention it to your pharmacist or doctor so they can keep an eye on your thyroid labs rather than you changing anything on your own.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Benvenga S, Ruggeri RM, Russo A, et al. Usefulness of L-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial. J Clin Endocrinol Meta
  2. 12761 Benvenga S, Amato A, Calvani M, Trimarchi F. Effects of carnitine on thyroid hormone action. Ann N Y Acad Sci 2004;1033:158-67. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

L-carnitine Also Interacts With Other Medications

L-carnitine has 18 moderate known drug interactions in our database — the Levothyroxine Sodium one above is one of them. If you take anything else, check it against the full list below.

Moderate 18 No known interaction 2,812 (incl. Levothyroxine Sodium)
Taking anything else?

Check Another of Your Medications Against L-carnitine

Covers the 2,830 medications we’ve checked against L-carnitine. Pick one to open its full report.

From our Q&A

Questions About L-carnitine or Levothyroxine Sodium

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers L-carnitine or Levothyroxine Sodium, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Levothyroxine Sodium

This page covers L-carnitine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Levothyroxine Sodium page lists 50 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Tiratricol (major), Acetyl-l-carnitine (moderate), Alpha-lipoic Acid (moderate) and Ascophyllum Nodosum (moderate).

Open the Levothyroxine Sodium interaction page

One more smart check

Taking L-carnitine as Part of a Dietary Supplement Product? Check It for Interactions

We hold 3,350 real supplement products containing L-carnitine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides L-carnitine. Your product may carry other ingredients that interact with Levothyroxine Sodium too, on top of L-carnitine — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where L-carnitine and Levothyroxine Sodium Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement L-carnitine
Top 10%

#81 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Levothyroxine Sodium
Top 5%

#126 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About L-carnitine

What is L-carnitine?

  • Exercise performance and recovery
  • Heart health
  • Weight management
  • Energy and fatigue
  • Male fertility
  • Carnitine deficiency

L-carnitine is a compound your body makes naturally and also gets from foods like meat. It helps cells turn fat into energy, and supplements are most clearly useful for people with a true carnitine deficiency. For most other uses, like weight loss or boosting athletic performance, the evidence is mixed or limited, so talk with your pharmacist or doctor before trying it.

Read the full L-carnitine monograph & all interactions
Keep checking

Other Drugs & L-carnitine

Browse every other medication we’ve checked against L-carnitine — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
18Moderate
0Minor
2,811No known interaction
18 Moderate 2,811 No interaction
Moderate

18 interactions

No interaction 2,811 drugs checked — show the list

We also checked L-carnitine against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,811 medications checked with no known interaction — see the full L-carnitine list.

See the full L-carnitine interactions page
See every supplement checked against Levothyroxine Sodium, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
L-carnitine monograph › Uses, safety, and every drug interaction in one place Levothyroxine Sodium supplement interactions › Every supplement checked against Levothyroxine Sodium, ranked by severity Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking L-carnitine and Levothyroxine Sodium Together: Common Questions

Can you take L-carnitine with Levothyroxine Sodium?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the L-carnitine and Levothyroxine Sodium interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Levothyroxine Sodium and L-carnitine?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.