Drug Interaction Report

Levothyroxine and Calcium Acetate: Interaction Details

A pharmacist-reviewed analysis of the Levothyroxine and Calcium Acetate interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Calcium Acetate

Calphron Phoslo PhosLo® Phoslyra Phoslyra®
+

Levothyroxine

Ermeza Euthyrox Levo-T Levo-T® Levothroid® Levoxyl Levoxyl® Synthroid
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Moderate
Can be significant — usually manageable with monitoring.
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.

Of 214 documented Levothyroxine interactions, 196 are rated moderate — including this one.
Onset
rapid
Evidence
probable
Severity
Moderate

What happens

Decreased levothyroxine absorption

Interaction Deep Dive

When levothyroxine was coadministered with calcium, exposure to levothyroxine was reduced by 20% to 25% in a randomized, crossover study (n=8)2. Administration of levothyroxine and calcium should be separated by at least 4 hours to avoid the possibility of chelation formation 1 and subsequent reduction of levothyroxine concentrations 2.

Why it happens (mechanism)

Formation of an insoluble chelate

Literature reports

1 report — tap to read

a) When levothyroxine was coadministered with calcium, exposure to levothyroxine was reduced by 20% to 25% in a randomized, crossover study (n=8). Healthy, euthyroid, normal volunteers received levothyroxine 1 mg orally plus calcium carbonate 1250 mg, calcium citrate 2381 mg, and calcium acetate 2001 mg on separate occasions with at least a 3-week washout between calcium salt forms. Each calcium preparation provided 500 mg of elemental calcium. The AUC of levothyroxine (measured as serum T4) when levothyroxine was coadministered with calcium carbonate, calcium citrate, and calcium acetate was 79.2%, 81.4%, and 75.1%, respectively, of the AUC of levothyroxine alone. However, there was a wide intersubject variation from 0% to 66% reduction in serum T4 when calcium was coadministered with levothyroxine 2.

Common questions

Can I take Levothyroxine and Calcium Acetate together?

Decreased levothyroxine absorption Always confirm with your pharmacist or prescriber before making any change.

How serious is the Levothyroxine and Calcium Acetate 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.

Questions for your pharmacist

  • Does my dose of Levothyroxine or Calcium Acetate 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 (2)

  1. Product Information: NOVOTHYROX(R) oral tablets, levothyroxine sodium oral tablets. Merck KGaA, Darmstadt, Germany, 2008. DailyMed
  2. Zamfirescu I & Carlson HE: Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid 2011; 21(5):483-486. PubMed
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Calcium Acetate

Keep reading about Calcium Acetate

All Calcium Acetate interactions (3) Calcium Acetate drug information
Also serious with: Ceftriaxone Injection Erdafitinib
Levothyroxine

Keep reading about Levothyroxine

All Levothyroxine interactions (214) Levothyroxine drug information
Also serious with: Furosemide Polystyrene Sulfonate Insulin Glargine, Recombinant Ferrous Sulfate
Beyond drug–drug

These medications also interact with supplements

Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:

Levothyroxine + Activated Charcoal Tiratricol Acetyl-l-carnitine Alpha-lipoic Acid Ascophyllum Nodosum Ashwagandha Bugleweed Calcium + 42 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.