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Can I take iron, calcium or magnesium with levothyroxine?

Yes, if you keep them 4 hours apart. The rule is on the levothyroxine label: iron and calcium supplements and antacids can cut how much of the tablet you absorb, and patients are told not to take it within 4 hours of them. When 20 adults took calcium carbonate with their levothyroxine for 3 months, average TSH rose from 1.6 to 2.7. Magnesium had a smaller effect in one small study. Your pharmacist can help plan the timing.

How well documented: On the drug’s label. The medicine’s FDA-approved label names this supplement, or the kind of product it is.

Last reviewed

How we grade evidenceEditorial policy

What's known

  • On the label. The Synthroid (levothyroxine) label says calcium carbonate and ferrous sulfate (a common iron supplement) may bind to levothyroxine, and that the two should be taken at least 4 hours apart. It warns that antacids, including those with magnesium hydroxide, can reduce absorption, and that less absorbed hormone can bring back an underactive thyroid. It tells patients not to take the tablet within 4 hours of iron or calcium supplements or antacids. (Synthroid label)
  • Calcium: drug-level studies. In 20 adults on a steady levothyroxine dose, adding 1,200 mg of calcium a day (as calcium carbonate) at the same time for 3 months pushed average TSH (the brain's signal to the thyroid, which rises when hormone runs low) from 1.6 to 2.7, and 20% went above the normal range. Levels recovered once the calcium ended. In 8 healthy adults, calcium carbonate, citrate and acetate each cut absorption by about 20% to 25%, so the form doesn't solve it. (Singh 2000, Zamfirescu 2011)
  • Iron: drug-level studies. In 14 people with an underactive thyroid, a ferrous sulfate tablet taken with their thyroxine each day for 12 weeks raised average TSH from 1.6 to 5.4, and 9 had more symptoms. In the lab, iron and thyroxine formed a complex that dissolves poorly. (Campbell 1992)
  • Magnesium: one small study. The label names magnesium only in antacids, not magnesium supplements. In 15 healthy adults, magnesium aspartate taken with levothyroxine cut absorption by 12%, and magnesium citrate by 7%, a change too small to be sure of. That is less than calcium or iron, but the authors still advise taking them apart. (Attinger 2025)

What to do

  • Talk to your prescriber or pharmacist before you add iron, calcium or magnesium, and tell them about antacids and multivitamins that contain these minerals.
  • The levothyroxine label says not to take the tablet within 4 hours of iron or calcium supplements or antacids. NIH's fact sheets repeat the 4-hour rule for calcium carbonate and iron. No label sets a rule for magnesium supplements, so ask your pharmacist. (ODS calcium, ODS iron)
  • If you start or quit one of these supplements, ask whether your TSH should be rechecked, especially if tiredness or other thyroid symptoms come back.
  • Don't stop or change a prescribed medicine on your own.

Sources

  1. US National Library of Medicine. Synthroid (levothyroxine sodium) tablets: prescribing information. DailyMed, revised 02/2024. DailyMed
  2. Singh N, et al. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000. PubMed 10838651 · doi:10.1001/jama.283.21.2822
  3. Zamfirescu I, et al. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid. 2011. PubMed 21595516 · doi:10.1089/thy.2010.0296
  4. Campbell NR, et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992. PubMed 1443969 · doi:10.7326/0003-4819-117-12-1010
  5. Attinger MC, et al. 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. PubMed 41221788 · doi:10.1111/cts.70409
  6. National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals. Updated 22 June 2026. ODS
  7. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. Updated 4 September 2025. ODS