Levothyroxine and Colesevelam: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Levothyroxine
Colesevelam
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced levothyroxine exposure and reduced efficacy of levothyroxine
Interaction Deep Dive
Concurrent use of levothyroxine and colesevelam may result in decreased bioavailability of levothyroxine. In a study, concurrent administration of colesevelam with levothyroxine significantly reduced the absorption of levothyroxine by approximately 96%3. Reductions were minimal in another study when levothyroxine was given 4 hours prior to colesevelam 4. Avoid concomitant use if possible. If coadministration is required, maximally separate administration 1 and administer levothyroxine at least 4 hours prior to colesevelam 42. Monitor for an adequate response to levothyroxine therapy and the levothyroxine dose should be adjusted appropriately 4.
Why it happens (mechanism)
Reduced absorption of levothyroxine by nonspecific binding with colesevelam
Literature reports
3 reports — tap to read
a) Concurrent administration of colesevelam and levothyroxine in participants significantly reduced levels of levothyroxine in one study. Coadministration of levothyroxine 600 micrograms and colesevelam 3.75 grams resulted in a 22% and 33% mean reduction in levothyroxine AUC and Cmax, respectively. Administration of levothyroxine 1 hour prior to colesevelam changed the AUC and Cmax by 6% and -2%, respectively. When levothyroxine was given 4 hours prior to colesevelam, the mean changes in AUC and Cmax were 1% and 8%, respectively 4.
b) There was an approximately 96% reduction in the absorption of levothyroxine when concomitantly administered with colesevelam, according to a study of 6 euthyroid healthy volunteers. Patients 27 to 40 years with normal serum thyroxine (T4) and TSH levels at baseline were administered levothyroxine sodium 1 mg orally at visit 1, followed by levothyroxine plus colesevelam 3.75 g or lanthanum carbonate 500 mg at 2 subsequent study visits at least 3 weeks apart. The mean increase of T4 AUC when levothyroxine was given alone was 1692 +/-183.5 mcg min/dL, but the increase was blunted when concomitant colesevelam was administered (107.5 +/- 45.8 mcg min/dL; 3.8% of the area of levothyroxine alone) 3.
c) Elevated TSH level was reported among patients receiving thyroid hormone replacement and colesevelam concomitantly during postmarketing use. Frequency and causality of these events have not been established 4.
Common questions
Can I take Levothyroxine and Colesevelam together?
Reduced levothyroxine exposure and reduced efficacy of levothyroxine Always confirm with your pharmacist or prescriber before making any change.
How serious is the Levothyroxine and Colesevelam interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Levothyroxine or Colesevelam need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (4)
- Centanni M, Duntas L, Feldt-Rasmussen U, et al: ETA guidelines for the use of levothyroxine sodium preparations in monotherapy to optimize the treatment of hypothyroidism. Eur Thyroid J 2025; 14(4):e250123. PubMed
- Product Information: TIROSINT(R)-SOL oral solution, levothyroxine sodium oral solution. IBSA Pharma Inc. (per FDA), Parsippany, NJ, 2023. DailyMed
- Weitzman SP, Ginsburg KC, & Carlson HE: Colesevelam hydrochloride and lanthanum carbonate interfere with the absorption of levothyroxine. Thyroid 2009; 19(1):77-79. PubMed
- Product Information: WELCHOL oral tablets, colesevelam hcl oral tablets. Daiichi Sankyo Inc, Parsippany, NJ, 2008. DailyMed
Keep reading about Levothyroxine
Keep reading about Colesevelam
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