Can I Take Magnesium With Synthroid (Levothyroxine)?

Levothyroxine (brand names Synthroid, Levoxyl, Tirosint, and others) is the synthetic form of T4, the FDA-approved replacement hormone for hypothyroidism. Magnesium is an over-the-counter mineral supplement sold as oxide, citrate, glycinate, aspartate, and several other salts. The two are not chemically incompatible, but taking them at the same time can lower how much levothyroxine your body absorbs.

The direct answer

Levothyroxine and magnesium can be taken by the same person on the same day, but not at the same time. Magnesium salts, like calcium and iron salts, can form insoluble complexes with levothyroxine in the acidic environment of the stomach, which lowers the amount of active hormone that reaches the bloodstream. The FDA-approved prescribing information for levothyroxine lists magnesium-containing antacids among the substances known to impair its absorption and recommends dosing them apart from levothyroxine. A 2025 open-label randomized crossover trial (the ThyroMag trial) directly tested levothyroxine co-administered with magnesium citrate and magnesium aspartate in healthy volunteers to characterize this interaction; the exact magnitude of absorption reduction reported in that trial should be checked against the published results before it is quoted as a specific number (ThyroMag trial, 2025). Until that verification is done, the safest and most defensible statement is qualitative: co-administration can meaningfully reduce absorption, and separation reduces that risk.

Why the interaction happens

Levothyroxine (T4) is a small molecule that depends on an acidic stomach environment to dissolve and on an intact upper small intestine to be absorbed. Divalent and trivalent cations, including magnesium, calcium, iron, and aluminum, are well documented to form chelate complexes with T4 when present in the gut at the same time. Complexed T4 is not absorbed and passes through unchanged. This mechanism is described in the FDA label for levothyroxine and is the same mechanism behind the well-known calcium-levothyroxine and iron-levothyroxine interactions.

Not all magnesium forms behave identically. Magnesium oxide and magnesium hydroxide (the forms in many antacids and laxatives) release a large amount of free magnesium ion in the stomach and are generally considered to carry the highest chelation potential. Magnesium citrate, glycinate, and aspartate are organic salts with different dissolution profiles, but all of them still deliver elemental magnesium capable of binding T4. There is no magnesium formulation that is established to be exempt from the separation precaution; the ThyroMag trial specifically compared citrate and aspartate forms against levothyroxine for this reason, and its results (once verified) should clarify whether the two organic salts differ meaningfully from each other.

The separation window

Levothyroxine guidance generally recommends taking mineral supplements, including magnesium, calcium, and iron, at least four hours apart from levothyroxine. In practice this means:

• Take levothyroxine first, on an empty stomach, with water only, and wait at least 30 to 60 minutes before eating.
• Take magnesium at a different part of the day: with lunch, with dinner, or at bedtime.
• If you take levothyroxine at bedtime instead of the morning, whether that is on empty stomach depends on timing relative to your last meal; move magnesium to the middle of the day instead, keeping the same four-hour buffer.

A single accidental overlap (taking both at the same time once) is not dangerous. It will reduce that day's levothyroxine absorption somewhat, but it does not justify doubling the next dose. Simply resume proper separation.

What is established, what is plausible, and what is not established

Evidence-status interaction assessment

What your clinician or pharmacist should verify before you act on this page: the specific magnesium form and dose you are taking, whether you are also on a proton pump inhibitor or another acid-reducing medication (which independently lowers levothyroxine absorption and depletes magnesium over time), your current TSH trend, and whether your levothyroxine brand or formulation has changed recently, since brand switches are a separate and common cause of TSH drift.

Monitoring after starting or changing magnesium

If you start a magnesium supplement while on levothyroxine, or if you have been taking both together without separating them, a TSH recheck around six to eight weeks later is a reasonable standard of practice, consistent with general guidance to recheck thyroid function after any change likely to affect levothyroxine absorption. If your TSH rises above your target range after starting magnesium, that is a signal the interaction is clinically active for you, not a signal to stop levothyroxine. Report new or worsening symptoms of underreplacement, such as returning fatigue, cold intolerance, or new constipation, to your prescriber rather than adjusting your dose on your own.

Standard serum magnesium testing reflects only a small fraction of total body magnesium and can look normal even when tissue stores are low. If there is clinical suspicion of magnesium depletion, for example from long-term PPI use or diuretic therapy, ask whether a different test or a symptom-based assessment is more appropriate. This is a judgment call for the ordering clinician, not something this page can resolve.

If you have already been taking both together

Do not stop levothyroxine or make a dose change on your own. A reasonable sequence, to discuss with your prescriber or pharmacist:

1. Start separating levothyroxine and magnesium by at least four hours, moving magnesium to midday or bedtime.
2. Keep your current levothyroxine dose unchanged for now.
3. Recheck TSH in about six to eight weeks.
4. If TSH drops toward target, that confirms the earlier overlap was reducing your effective dose. Your prescriber may choose to leave your levothyroxine dose as is or adjust it based on the new value; this is an individualized decision, not something to infer from a general article.

A simple log of when you take each medication for the first couple of weeks can help your care team troubleshoot if TSH does not move as expected.

When to seek care sooner than a routine recheck

Contact your prescriber promptly, rather than waiting for a scheduled recheck, if you develop chest pain, a rapid or irregular heartbeat, severe swelling, or signs of a thyroid storm or myxedema (very rare, but the risk is higher in anyone with poorly controlled thyroid disease). Seek urgent care for these symptoms rather than adjusting supplements or thyroid medication at home.