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Medicines and supplements

Can you take magnesium citrate with levothyroxine?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Ask your pharmacist or doctor before combining. The levothyroxine label lists antacids, such as those made with aluminum and magnesium hydroxides, among products that may reduce its absorption. In a 2025 study of 15 healthy adults, magnesium aspartate taken at the same time lowered levothyroxine absorption by 12%; the 7% drop with citrate could have been chance.1,2

Key facts

  • The label explains that levothyroxine needs stomach acid to be absorbed well, and that antacids, acid reducers and sucralfate can lower that acid.1
  • MedlinePlus lists antacids among the nonprescription products that may interact with levothyroxine.3
  • The 2025 study used powdered magnesium dissolved in water: 301 mg of magnesium as citrate or 243 mg as aspartate.2
  • The label also names calcium and iron supplements as products that can lower levothyroxine absorption.1

Ask your pharmacist how far apart to take them

The levothyroxine label lists antacids, such as those made with aluminum and magnesium hydroxides, among products that may reduce levothyroxine absorption by lowering stomach acid. In a small 2025 study, magnesium aspartate taken at the same time lowered levothyroxine absorption by 12%, and magnesium citrate by 7%, a drop too small to rule out chance.1,2,3

On the medicine's label

What is documented

Levothyroxine's prescribing information has a table of drugs that may decrease its absorption. Antacids are on it, with aluminum and magnesium hydroxides given as examples, next to proton pump inhibitors and sucralfate. The label's reason is acid: levothyroxine needs an acidic stomach to be absorbed well, and these products can raise stomach pH.1

MedlinePlus includes antacids in its list of nonprescription products that may interact with levothyroxine, and asks you to tell your doctor and pharmacist about every vitamin and supplement you take.3

What a 2025 study found

The ThyroMag trial gave 15 healthy adults a single 1 mg dose of levothyroxine on three visits: alone, with 301 mg of magnesium as citrate, and with 243 mg as aspartate, each powder dissolved in water. Aspartate lowered the levothyroxine absorbed by 12%. Citrate lowered it by 7%, a difference too small to rule out chance.2

The authors found the effect smaller than reports for calcium and iron, and said their results may not apply to magnesium tablets or capsules. They still advised people on levothyroxine to take it apart from magnesium products.2

Citrate, oxide and other forms

Magnesium oxide is sold as an antacid as well as a supplement. One 400 mg antacid tablet carries 241.3 mg of magnesium, and its Drug Facts label says antacids may interact with certain prescription drugs.4

Glycinate, malate and L-threonate were not tested with levothyroxine in the sources we found. Nothing shows that any form escapes the effect, so ask about the one you use.

Why it matters

Levothyroxine replaces thyroid hormone when the thyroid is underactive, and its dose is adjusted by blood tests. Anything that changes how much is absorbed can shift those results, which is why the label tells prescribers to monitor people who take these products with it.1

Questions for your pharmacist

  • How should I time levothyroxine around magnesium, calcium and iron?
  • Is my antacid one of the products on the label's list?
  • Should my thyroid levels be checked after I start or stop magnesium?
  • Does my acid reducer affect my levothyroxine as well?

Bring every supplement bottle and antacid you use, since a multivitamin or electrolyte drink may also contain magnesium.

Questions people ask

Can I take magnesium glycinate with levothyroxine?

Glycinate was not tested. The label and the 2025 study point to magnesium products in general, so ask your pharmacist how to time them.1,2

Is magnesium as much of a problem as calcium or iron?

In the 2025 study the effect was smaller than reported for calcium and iron, but the study was small and used single doses, and the label lists antacids alongside them. Ask your pharmacist.1,2

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Sources

  1. Synthroid (levothyroxine sodium) tablets: prescribing information (AbbVie). DailyMed, U.S. National Library of Medicine. Label effective February 20, 2024. Opened 2026-10-07.
  2. Attinger MC, von Felten S, Rodrigues CL, Krützfeldt J, Risch L, Bonzon J. Single Center, Open-Label, Randomized Crossover Trial on Drug-Drug Interactions of Levothyroxine/Magnesium-Citrate and Levothyroxine/Magnesium-Aspartate in Healthy Subjects: The ThyroMag Trial. Clin Transl Sci. 2025;18(11):e70409. Opened 2026-10-07.
  3. Levothyroxine. MedlinePlus Drug Information, U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
  4. Magnesium Oxide 400 mg tablets, antacid: Drug Facts label (Westminster Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Label effective July 6, 2026. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.