Levothyroxine is unusually sensitive to what else is in your stomach. Its label says to take it on an empty stomach and to keep several common supplements hours away from it, because minerals can bind to it in the gut and leave less to absorb. Magnesium is one of those minerals. The known issue with magnesium is absorption, not an effect on the thyroid itself, which makes this mostly a question of timing.

This article is educational and is not medical advice. Don't change how or when you take levothyroxine based on a web article. If you take thyroid medication, check with your prescriber or pharmacist before adding magnesium or changing when you take it.

What the levothyroxine label says

The U.S. prescribing information for Synthroid, a widely used brand of levothyroxine, says to take it once a day on an empty stomach, half an hour to an hour before breakfast, and at least four hours before or after drugs known to interfere with its absorption. It names calcium carbonate and ferrous sulfate (a common iron supplement) as products to keep at least four hours apart, and lists antacids containing aluminum and magnesium hydroxides among the products that can reduce absorption. Magnesium supplements are not named on their own.

The first trial of magnesium itself

Until 2025, nobody had measured magnesium's effect directly. The ThyroMag trial gave 15 healthy adults a single 1 mg test dose of levothyroxine on three occasions: alone, with magnesium aspartate (243 mg of magnesium), and with magnesium citrate (301 mg of magnesium), each taken at the same time as the levothyroxine as a powder dissolved in water. With magnesium aspartate, the amount of thyroxine absorbed fell by 12%. With magnesium citrate it fell by 7%, a difference that was not statistically significant (Attinger 2025).

The authors noted that these effects were smaller than those reported for calcium and iron. For comparison, in an earlier study 20 people on stable levothyroxine took 1,200 mg of calcium a day as calcium carbonate with their dose for three months. Their average TSH rose from 1.6 to 2.7, and one in five went above the normal range (Singh 2000). The ThyroMag authors judged that magnesium's effect may not matter clinically for most people, cautioned that their result applies to dissolved powders and can't be extended to tablets, and still advised taking levothyroxine separately from magnesium, especially when TSH needs to stay in a narrow range.

Case reports: when it showed up in blood tests

A 1999 Danish report described two people on an unchanged levothyroxine dose whose TSH rose markedly and whose thyroxine fell, one while taking an aluminum hydroxide antacid and the other a magnesium oxide laxative. Both recovered after stopping. In the same paper's laboratory tests, an antacid mix of aluminum hydroxide, magnesium hydroxide and magnesium carbonate bound levothyroxine, but magnesium oxide on its own did not (Mersebach 1999), so the magnesium oxide case isn't fully explained. Two case reports can't show how often this happens. They show that it can.

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Over weeks: one clinic study

One study has looked beyond a single dose. A 2026 Turkish clinic study reviewed the records of 45 people with Hashimoto's thyroiditis and low blood magnesium who took magnesium oxide (365 mg of magnesium a day) for two months while their levothyroxine dose stayed the same. Their median TSH rose from 1.73 to 2.3 mIU/L, a statistically significant change that stayed within the normal range, while free T4 didn't change (Urhan 2026). It was a retrospective review with no comparison group, and the records didn't show how far apart people took the two, so it can't prove magnesium caused the rise. The author suggests reduced absorption, or missed doses, as possible explanations.

What hasn't been studied

We found no trial that has tested the forms most often sold as supplements, such as magnesium glycinate, malate or L-threonate, with levothyroxine, or that measured whether spacing the doses prevents a change in TSH (October 2026). The reasonable working assumption, and the one both the ThyroMag and Urhan authors made, is to keep the two apart.

How to space them

  • Take levothyroxine exactly as prescribed, usually first thing in the morning on an empty stomach, and take magnesium later in the day. The label's four-hour gap for calcium and iron is a sensible yardstick to discuss with your pharmacist, and evening magnesium is an easy way to get there.
  • If you take levothyroxine at bedtime, the same principle applies the other way round: ask your pharmacist about taking magnesium earlier in the day rather than taking the two together.
  • Check every product, not just the magnesium bottle. Multivitamins often combine magnesium with calcium and iron, both on the label's four-hour list, and magnesium is also in many antacids and laxatives, including milk of magnesia and magnesium citrate.
  • Keep your routine consistent. Your dose was adjusted to your current habits, so tell your prescriber when you start or stop a daily magnesium supplement, and ask whether a TSH recheck makes sense.
  • If you have kidney disease, don't take magnesium supplements without your clinician's advice. Magnesium is cleared by the kidneys.

Does magnesium help the thyroid?

Some articles suggest magnesium for thyroid health. The evidence usually cited is observational: in a Chinese study of 1,257 people, those with severely low blood magnesium were more likely to have hypothyroidism and Hashimoto's thyroiditis (Wang 2018). A finding like that can't show that low magnesium causes thyroid problems or that supplements help, and it is no reason to take magnesium at the same time as levothyroxine.

The bottom line

The concern with magnesium and levothyroxine is absorption: taken together, magnesium can reduce how much of your dose you absorb. The one direct trial found a modest effect, a small clinic study saw TSH drift up over two months, the case reports show it can occasionally matter, and the practical answer is simple. Keep them hours apart, keep your routine consistent, and tell your prescriber about any supplement you take every day.

References

  • Attinger MC, von Felten S, Rodrigues CL, 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." Clinical and Translational Science, 2025;18(11):e70409. PMID: 41221788.
  • Singh N, Singh PN, Hershman JM. "Effect of calcium carbonate on the absorption of levothyroxine." JAMA, 2000;283(21):2822–2825. PMID: 10838651.
  • Mersebach H, Rasmussen AK, Kirkegaard L, Feldt-Rasmussen U. "Intestinal adsorption of levothyroxine by antacids and laxatives: case stories and in vitro experiments." Pharmacology & Toxicology, 1999;84(3):107–109. PMID: 10193669.
  • Urhan E. "Effects of magnesium coadministration on thyroid function and autoantibody titers in euthyroid Hashimoto's thyroiditis." Cumhuriyet Medical Journal, 2026;48(1):8–13. doi:10.7197/cmj.1879425.
  • Wang K, Wei H, Zhang W, et al. "Severely low serum magnesium is associated with increased risks of positive anti-thyroglobulin antibody and hypothyroidism: a cross-sectional study." Scientific Reports, 2018;8(1):9904. PMID: 29967483.
  • AbbVie. "Synthroid (levothyroxine sodium) tablets: prescribing information." DailyMed, label version of February 2024. dailymed.nlm.nih.gov