Magnesium, Zinc and Levothyroxine: How Far Apart to Take Them

You can take magnesium or zinc while you are on levothyroxine. The rule is about timing, not about avoiding the minerals: keep them at least 4 hours away from your thyroid pill, the same window the levothyroxine label uses for calcium, iron and antacids [C1]. For most people that simply means levothyroxine when you wake up and minerals with dinner or before bed.

Can you take magnesium if you take levothyroxine?

Yes. Nothing in the levothyroxine label forbids magnesium [C1]. The issue is narrower: magnesium taken at the same moment as your tablet can stop part of the dose from being absorbed [C2]. Levothyroxine is dosed in micrograms [C1], so even a modest loss, repeated every day, can matter over weeks.

Whether magnesium does anything for thyroid function itself is covered in magnesium and the thyroid. This page is only about timing.

Why do minerals interfere with levothyroxine?

Minerals with a double (or triple) positive charge, such as calcium, iron, magnesium and aluminium, can bind levothyroxine in the gut. The ThyroMag researchers describe this as complexing: the mineral and the hormone stick together, and the bound hormone is not absorbed [C2]. When iron and thyroxine were mixed in a test tube, a poorly soluble purple complex formed [C5].

The best-studied examples are calcium and iron. In adults with normal thyroid function, calcium carbonate, citrate and acetate each cut levothyroxine absorption by about 20-25% when swallowed together [C4]. In 14 people on stable thyroid replacement, taking a 300 mg ferrous sulfate tablet with their dose every day for 12 weeks raised average TSH from 1.6 to 5.4 mU/L [C5]. A 2021 systematic review of 63 studies found calcium and iron supplements consistently lowered absorption and named spacing as an effective fix [C6].

What the levothyroxine label actually says

The US prescribing information for Synthroid (levothyroxine) says to take it on an empty stomach, half an hour to one hour before breakfast, and "at least 4 hours before or after drugs known to interfere" with its absorption [C1]. Its interaction table lists antacids "(e.g., aluminum & magnesium hydroxides, simethicone)" among products that can reduce absorption, and its patient counselling section tells people not to take the tablets within 4 hours of iron supplements, calcium supplements and antacids [C1].

The label does not name magnesium supplements or zinc [C1]. What follows on those two rests on smaller studies and shared chemistry.

How strong is the evidence for magnesium?

Weaker than for calcium and iron. Until 2025, it came only from antacids and laxatives. In a 1999 Danish report, two people on a fixed levothyroxine dose developed a marked rise in TSH and low thyroxine while taking aluminium hydroxide (for indigestion) or magnesium oxide (for constipation); TSH came back down after they stopped [C3]. In the same paper's lab tests, an antacid mix of aluminium hydroxide, magnesium hydroxide and magnesium carbonate bound levothyroxine in a dose-related way, while magnesium oxide alone did not [C3]. The clinical signal was real; the lab results for magnesium oxide were mixed.

In ThyroMag, the first trial of magnesium supplements (2025), fifteen adults with normal thyroid function took a single 1 mg test dose of levothyroxine alone, with magnesium aspartate, or with magnesium citrate, in random order [C2]. Taken together, magnesium aspartate lowered total T4 absorption by 12%, and magnesium citrate by 7%, a difference that did not reach statistical significance [C2].

The authors called these effects smaller than those described for calcium and iron, possibly because the magnesium was given as a liquid [C2]. They still advised people with hypothyroidism to separate levothyroxine from magnesium products, especially when TSH needs to stay in a narrow range [C2]. Keep the limits in mind: one small study, volunteers without thyroid disease, one dose, no TSH follow-up.

Which magnesium is best if you take levothyroxine?

No form has been shown to be safe to swallow together with levothyroxine, so the timing rule applies to all of them.

• Citrate and aspartate: the only two forms tested with levothyroxine; citrate had the smaller effect [C2]. The NIH notes that aspartate, citrate, lactate and chloride are absorbed more completely than oxide and sulfate [C9].
• Oxide: a common supplement form [C9], also used as a laxative; it was the product in one of the 1999 TSH cases [C3]. The NIH lists oxide, carbonate, chloride and gluconate as the forms most often reported to cause diarrhoea [C9].
• Hydroxide: the active ingredient in milk of magnesia (about 500 mg of magnesium per tablespoon) and in some heartburn tablets [C9]. Antacids with magnesium hydroxide are named on the levothyroxine label [C1].
• Glycinate (bisglycinate): we found no study testing it with levothyroxine. Assume the same 4-hour gap.

Whatever the form, the NIH upper limit for magnesium from supplements and medicines is 350 mg a day for adults, unless your doctor advises otherwise [C9]. Magnesium from food is not counted toward that limit [C9].

What about zinc, multivitamins and selenium?

Zinc. We found no published human study of zinc taken together with levothyroxine, and the label is silent on it [C1]. Zinc carries the same double positive charge as calcium, iron and magnesium, so binding is plausible, but it has not been measured. Separating zinc by 4 hours costs nothing. For what zinc does for the thyroid, see zinc and thyroid function.

Multivitamins with minerals. Many combine several of these minerals at once. If yours contains calcium or iron, the label's 4-hour instruction applies directly [C1]. Check the label for biotin too: the levothyroxine label advises stopping biotin-containing supplements at least 2 days before TSH or T4 blood tests [C1].

Selenium. Selenium is not one of the minerals named in the binding studies, and none of our sources lists a selenium-levothyroxine interaction [C1][C6]. In a 12-month trial of 412 people on levothyroxine, 200 µg of selenium a day did not change the levothyroxine dose they needed [C8]. The catch: many "thyroid support" products pair selenium with zinc, and then the zinc rule applies.

How far apart should each supplement be from levothyroxine?

Supplement or product	Common spacing from levothyroxine	What the evidence is
Magnesium supplement (citrate, glycinate, oxide…)	At least 4 hours	One small 2025 crossover trial: 7-12% lower T4 absorption when taken together [C2]. Interval itself not tested.
Antacid or laxative with magnesium (hydroxide, oxide, carbonate)	At least 4 hours	Named on the levothyroxine label [C1]; TSH rose in case reports and came back down after stopping [C3].
Zinc supplement	At least 4 hours (precaution)	No human study with levothyroxine; spacing based on similar mineral chemistry.
Calcium supplement	At least 4 hours	Label instruction [C1]; about 20-25% lower absorption with carbonate, citrate or acetate [C4].
Iron supplement	At least 4 hours	Label instruction [C1]; TSH rose from 1.6 to 5.4 mU/L in a 12-week study [C5].
Multivitamin with minerals	At least 4 hours	Applies through its calcium and iron [C1][C6]. Stop biotin 2 days before thyroid labs [C1].
Selenium (alone)	No specific spacing known	No binding interaction listed in our sources [C1][C6]; 200 µg/day did not change levothyroxine needs over 12 months [C8].

The 4-hour figure is the label's number for the products it names [C1]. For magnesium and zinc supplements no interval has been tested, so 4 hours is a conservative choice, not a measured minimum. More combinations are in our supplement timing guide and the calcium and iron 4-hour rule.

A simple daily schedule

If you take levothyroxine in the morning (the label's standard instruction [C1]):

• On waking: levothyroxine with water, on an empty stomach.
• 30-60 minutes later: breakfast and coffee.
• Afternoon, evening or bedtime: magnesium, zinc, calcium, iron or a multivitamin, at least 4 hours after the tablet.

If you also take iron or calcium, ask your pharmacist how best to split them across the day.

If you take levothyroxine at bedtime, the schedule flips. A randomized crossover trial in 105 people (90 analysed) found bedtime dosing lowered TSH by about 1.25 mIU/L compared with morning dosing, with no change in quality of life [C7], and a systematic review found morning and bedtime intake equally effective [C6]. In that case, take your minerals in the morning or at lunch, so at least 4 hours separate them from the bedtime dose. Moving your levothyroxine time is a decision for your doctor; our bedtime dosing guide explains who it tends to suit.

Whatever you choose, keep it the same every day. Your dose is judged by periodic TSH and T4 checks [C1], and a stable routine makes those numbers meaningful. Thyra's medication timing feature lays out your coffee, breakfast and supplement windows around your levothyroxine time, which helps if your mornings vary.

When to talk to your doctor

Tell your doctor or pharmacist before your next TSH test if you have:

• started, stopped or changed the timing of magnesium, zinc, calcium, iron or a multivitamin;
• been using antacids or magnesium laxatives regularly (people often forget to mention these [C3]);
• started an acid-reducing medicine, which can also affect absorption [C1];
• noticed tiredness, feeling cold or other hypothyroid symptoms returning after a supplement change [C5].

Do not change your levothyroxine dose on your own to make up for a supplement. If your levels shift, your doctor can decide whether to adjust the timing, the supplement or the dose.

Frequently asked questions

Can I take magnesium at night if I take levothyroxine in the morning?

Yes. It is the easiest arrangement, because the gap is then far longer than the 4 hours the label uses for antacids, calcium and iron [C1]. Keep the time similar from day to day.

Can I take zinc and levothyroxine together?

It is better not to. Zinc has not been tested with levothyroxine, but it is the same kind of mineral as calcium, iron and magnesium, which do reduce absorption [C2][C4][C5]. Taking zinc at least 4 hours from your tablet is a simple precaution.

Is magnesium bad for the thyroid?

None of the sources we reviewed shows magnesium harming the thyroid. The only documented problem is reduced absorption of levothyroxine when both are swallowed together [C2][C3]. Spacing them avoids that overlap, and staying under 350 mg a day from supplements keeps you within the NIH limit [C9].

My levothyroxine tablet contains magnesium stearate. Is that a problem?

No. Magnesium stearate is a listed inactive ingredient of the tablet itself [C1]. It is part of the approved product, which is a different situation from swallowing a magnesium supplement with your dose.

What if I accidentally took magnesium with my levothyroxine?

One occasional overlap is unlikely to matter much; in ThyroMag, a single overlap reduced absorption by 7-12% [C2]. Go back to your usual spacing the next day. If it happens often, mention it before your next TSH check.

Bottom line

Magnesium and zinc fit fine alongside levothyroxine as long as they are not swallowed together. Take levothyroxine on an empty stomach and the minerals at least 4 hours later, usually in the evening, or in the morning if your doctor has you on bedtime dosing [C1][C7]. Keep the routine steady and tell your doctor about any supplement change before your next TSH test.