The short answer
Taking magnesium at the same time as levothyroxine can reduce thyroid-hormone absorption. The direct study tested dissolved magnesium aspartate and citrate, not every magnesium product. Keep interfering products apart under your prescription instructions; the SYNTHROID label specifies four hours from antacids.
What this answer covers
Oral levothyroxine tablets with oral magnesium products. The label specifically addresses magnesium-containing antacids; the trial tested dissolved aspartate and citrate. It did not test glycinate, all capsules or injected thyroid hormone.
- Supplement or preparation
- magnesium
- Medication ingredient
- levothyroxine
What did the newer study show?
In fifteen healthy adults, magnesium aspartate reduced six-hour thyroid-hormone exposure by about 12%. The citrate estimate was a smaller 7% reduction and was not statistically significant. Both were powders dissolved in water, taken with a large single levothyroxine dose.
Can I use those numbers for magnesium glycinate?
No. Different salts and dosage forms were not compared comprehensively. The study does not give a measured reduction for glycinate, oxide tablets or your particular capsule. It also did not measure long-term thyroid control.
What does the four-hour advice mean?
SYNTHROID counseling says not to take the tablets within four hours of antacids, including the magnesium-containing products discussed in its interaction section. Its general directions also separate medicines known to impair absorption. A pharmacist can apply these instructions to your supplement; the magnesium experiment itself did not test a four-hour gap.
Does this mean stopping magnesium?
Ask the clinician or pharmacist to review why you take it, the amount and the schedule. A change in supplement use can matter when interpreting thyroid tests. Do not change your thyroid dose yourself or assume that switching magnesium salts guarantees consistent absorption.
Evidence and practical considerations
Absorption & effectiveness
Taking magnesium at the same time as levothyroxine can reduce thyroid-hormone absorption. The direct study tested dissolved magnesium aspartate and citrate, not every magnesium product. Keep interfering products apart under your prescription instructions; the SYNTHROID label specifies four hours from antacids.
- Exact scope
- Oral levothyroxine tablets with oral magnesium products. The label specifically addresses magnesium-containing antacids; the trial tested dissolved aspartate and citrate. It did not test glycinate, all capsules or injected thyroid hormone. with Levothyroxine active moiety, RxCUI 10582 (IN), supplied as oral levothyroxine sodium tablets. The DailyMed active-ingredient table explicitly connects the sodium salt to levothyroxine. No extrapolation to injectable hormone or another thyroid drug.
- Medication form and route
- oral levothyroxine tablets with oral supplement
What remains uncertain
- A small single-dose study in healthy volunteers supports a formulation-limited absorption effect. Antacid labeling supplies practical separation advice; no trial established a preventive interval for each magnesium salt.
Factors that may matter: exact product and dose; thyroid condition and treatment target; administration routine.
Read the supporting source summaries
Source 1: regulatory prescribing information
rxabbvie.com · Source check Sep 10, 2026
Population: Patients prescribed oral levothyroxine sodium tablets.
Addresses fasting administration, interfering antacids and food, thyroid monitoring and the risks of over- or undertreatment.
How this applies: Oral levothyroxine tablets with oral magnesium products. The label specifically addresses magnesium-containing antacids; the trial tested dissolved aspartate and citrate. It did not test glycinate, all capsules or injected thyroid hormone. The source supports medication administration and monitoring, not an isolated effect for every supplement.
Does not quantify every supplement interaction; recommendations for these tablets do not establish identical behavior for liquid or injected levothyroxine.
Source 2: primary clinical research
ascpt.onlinelibrary.wiley.com · Source check Sep 10, 2026
Population: Fifteen healthy adults in a randomized crossover study.
Six-hour T4 exposure fell 12% with aspartate; the 7% citrate reduction was not statistically significant.
How this applies: Oral levothyroxine tablets with oral magnesium products. The label specifically addresses magnesium-containing antacids; the trial tested dissolved aspartate and citrate. It did not test glycinate, all capsules or injected thyroid hormone. Apply only the preparation, population and limitations recorded for this source.
Dissolved powders and a large single thyroid-hormone dose; no glycinate, magnesium tablets, long-term TSH outcome or separation-interval comparison.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Original research, current labeling and relevant guidance checked. Consumer article written with preparation-specific limits and proposed corrections to checker statements. | |
| Research summary published |
Article revision: 70e48b66a5bf6235
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.