Supplements and Levothyroxine: How Long to Wait for Iron, Calcium and Magnesium

Take levothyroxine with water on an empty stomach, then wait at least 30 minutes, and preferably 60, before food. Iron, calcium and antacids need a much longer gap, about 4 hours, because they bind the tablet in the gut. The Australian label says magnesium supplements should be kept several hours away. Vitamin D on its own is not known to interfere, though many vitamin D products also contain calcium. If your doctor has you take it at bedtime instead, take it at least three hours after your last meal.

Many people are told the first rule and never hear the second. Here is which supplements interfere, how far apart to keep them, and how strong the evidence behind each number is.

Two different waits: food versus mineral supplements

Levothyroxine has two separate timing rules, and they are easy to mix up.

• Food needs a head start of 30 to 60 minutes. The Australian Eutroxsig leaflet says to take the tablet first thing in the morning on an empty stomach, at least 30 minutes and preferably 60 minutes before any food. In a crossover trial, taking it with breakfast instead gave a higher and more variable TSH.
• Concentrated minerals need a separation of hours. The Australian product information lists calcium, aluminium, magnesium and iron supplements, soya flour and reflux medicines called proton pump inhibitors as things that interfere with absorption, and says they should be separated from the tablet by several hours. The usual working figure is about 4 hours: the US label for one brand says at least 4 hours for iron and calcium.

So breakfast does not need to wait 4 hours. An iron or calcium tablet does.

Food needs a head start of 30 to 60 minutes. Iron and calcium need a separation of hours. They are two different rules.

How long to wait after levothyroxine for each supplement

This table assumes you take your tablet in the morning. The gaps work in both directions: a supplement taken shortly before the tablet can interfere too.

Supplement	Why it interferes	How far apart
Iron	Binds the hormone in the gut so less is absorbed	About 4 hours
Calcium, including calcium with vitamin D	Binds the hormone; carbonate, citrate and acetate all did it in a small study	About 4 hours
Antacids	Bind the hormone in the gut	About 4 hours
Magnesium supplements	Reduce absorption, by a smaller amount than iron or calcium	Several hours
Multivitamins with minerals	Usually contain iron, calcium or both	Same as the minerals in them: several hours
Vitamin D on its own	Not listed as an interaction on the Australian label	No set gap; check the label for added calcium
Protein shakes	It is food; added minerals or soy also interfere	At least 30 minutes, ideally 60; several hours if it has added minerals or soy protein
Soy products and soy protein	Lower absorption	Several hours; tell your doctor when you start or stop
Fibre supplements	Can hold onto the hormone in the gut	Keep them apart; tell your doctor
Reflux medicines (proton pump inhibitors)	Lower stomach acid, which the tablet needs	The label says several hours, but the effect lasts all day; tell your doctor when you start or stop
Coffee	Lowers absorption if taken with the tablet	At least 30 minutes, ideally 60, like breakfast

Sources: the Australian Eutroxsig and Oroxine product information, plus the studies discussed below.

Coffee has its own guide: coffee and thyroid medication, how long to wait.

Why iron and calcium need hours, not minutes

Both bind thyroxine in the gut. The research is small but consistent.

In a 1992 study, 14 people on a stable thyroxine dose took an iron tablet (ferrous sulfate) at the same time as their thyroxine for 12 weeks. Their average TSH rose from about 1.6 to about 5.4, and 9 of the 14 developed more symptoms or signs of an underactive thyroid. In a 2000 study, 20 people took calcium carbonate with their tablet for three months. Average TSH rose from 1.6 to 2.7, and about 1 in 5 went above the normal range. A later study found calcium carbonate, calcium citrate and calcium acetate each cut absorption by about 20 to 25%, so switching to a different type of calcium does not get around it.

Why about 4 hours? The exact figure has never been tested head to head. The American Thyroid Association's guideline calls a 4-hour separation "traditional, but untested", and the Australian product information simply says several hours. Think of 4 hours as a sensible working number, not a precise threshold.

If you have been advised to take iron, space it rather than skip it. See our guide to iron, ferritin and the thyroid.

Magnesium, vitamin D and multivitamins

Magnesium

The Australian product information lists magnesium supplements among the things to keep several hours away from your tablet. The first study to measure it directly, in 2025, gave 15 healthy adults a single dose of levothyroxine with or without magnesium. Magnesium aspartate lowered absorption by about 12% and magnesium citrate by about 7%, which was not statistically clear. That is smaller than the effect of calcium or iron, but the authors still advised keeping the two apart.

Vitamin D

Vitamin D on its own does not appear on the Australian label's interaction list, and no study was found showing that it blocks absorption. The catch is that many vitamin D products are combined with calcium. Read the label: if it contains calcium, it follows the calcium rule.

Multivitamins

Many multivitamins contain iron, calcium, zinc or magnesium, so they follow the same rule as the minerals inside them: keep them several hours from your tablet. The Australian product information puts it the same way: calcium, iron, magnesium and aluminium supplements go several hours apart from the tablet. Biotin, common in multivitamins, can also distort some thyroid test results; see our guide to supplements to be careful with on an underactive thyroid.

Protein shakes, soy and fibre

A plain protein shake is food, so it follows the food rule: at least 30 minutes, ideally 60, after the tablet. In a small study, a glass of milk taken with the tablet lowered absorption by about 8%, likely because of its calcium. A shake with added minerals counts as a mineral product and goes several hours away. Our guide to protein powder with Hashimoto's or levothyroxine goes through this in detail.

Soy is named on the Australian label. One published case described a person who drank a soy protein shake straight after her levothyroxine and needed an unusually high dose; once the two were separated, a lower dose worked. The Australian label also notes that a dose adjustment may be needed when a soy supplement is started or stopped, which is why it is worth telling your doctor.

Fibre is less clear. The Australian label says high-fibre diets can reduce absorption, and a small 1996 study found some people needing unusually high doses needed less thyroxine once a fibre supplement was withdrawn. There is no need to avoid fibre-rich food; just keep a fibre supplement apart from your tablet.

What about antacids and reflux medicines?

Antacids containing aluminium hydroxide or calcium carbonate bind the hormone in the same way as mineral supplements. In one case report, a person stable on levothyroxine had persistently high TSH while using an aluminium antacid, which settled once it stopped. Keep antacids about 4 hours from your tablet.

Proton pump inhibitors, the daily reflux medicines, work differently. The Australian label lists them with the products to keep several hours apart, but they lower stomach acid for the whole day, so spacing alone may not be enough. A 2021 systematic review of seven studies found that each one showed TSH rising after a proton pump inhibitor was added. If you start or stop one, tell your doctor, as your thyroid test may need checking.

Morning or night dosing: how the gaps change

Morning is not the only option. The American Thyroid Association's guideline says the tablet should be taken consistently, either 60 minutes before breakfast or at bedtime, three or more hours after the evening meal. In a crossover trial of 90 people, bedtime dosing gave a lower TSH than morning dosing 30 minutes before breakfast, though how people felt did not change. Whether bedtime suits you is a question for your doctor, not something to switch on your own.

Whichever time you use, keep the routine the same each day so your test results reflect how you really take it. If you are not on thyroid medication, none of these timing rules apply.

What if I forget my tablet or take a supplement too close to it?

The Eutroxsig leaflet is clear about a missed dose: if it is almost time for your next dose, skip the one you missed and take the next one as usual. Otherwise, take it as soon as you remember, then go back to your normal routine. Do not take a double dose to make up for it. If you are unsure, ask your pharmacist.

One morning where iron or calcium landed too close to the tablet is unlikely to matter much on its own: the rises in TSH in the studies above built up over weeks of taking the two together. Get back to your usual spacing the next day. If it happens often, mention it to your doctor or pharmacist, because it can show up in your next test result.

Does this apply after thyroidectomy and with Hashimoto's?

Yes. These rules apply to anyone who takes levothyroxine, whether your thyroid was removed or you have Hashimoto's or an underactive thyroid for another reason. The tablet behaves the same way in the gut.

After a total thyroidectomy there is one extra layer: some people are prescribed calcium or calcitriol because the parathyroid glands were affected during surgery. Do not stop or change prescribed calcium to make the timing easier. Ask your surgical team or pharmacist how to space it around your tablet. Our guide to supplements after thyroidectomy covers calcium after surgery in more detail.

Whatever your situation, tell your doctor or pharmacist when you start or stop iron, calcium, soy, a fibre supplement or a reflux medicine. That is when a dose is most likely to need checking.

Where ThyroBase fits

ThyroBase is a daily morning and evening nutrition powder, a food rather than a medicine, that sits alongside your thyroid medication, never instead of it. Both serves contain minerals, so they follow the supplement rule, not the food rule. If you take your tablet in the morning, have the AM serve about 4 hours after it. If you take it at bedtime, the gap moves to the evening: have the PM serve with or before your evening meal so it is well clear of the tablet. If you are not on medication, the AM serve can go with breakfast. The morning serve contains added vitamin D, so if you are on prescribed calcium or calcitriol, ask your team before starting. Here is who ThyroBase is, and is not, designed for.

Frequently asked questions

How long should I wait after levothyroxine before taking iron?

The usual advice is about 4 hours. The Australian product information says iron supplements should be separated from the tablet by several hours, and a study of iron taken with thyroxine found TSH rose from about 1.6 to 5.4 over 12 weeks.

How long after levothyroxine can I take calcium?

About 4 hours is the usual working gap, and the Australian label says several hours. Calcium carbonate, citrate and acetate all reduced absorption in a small study, so changing the type does not avoid the problem. If you take prescribed calcium after thyroid surgery, do not stop it; ask your team how to space it.

Can I take magnesium with levothyroxine?

Not at the same time. The Australian label lists magnesium supplements among the things to keep several hours away from your tablet, and a 2025 study found magnesium lowered absorption, though by less than calcium or iron.

Can I take vitamin D with levothyroxine?

Vitamin D on its own is not listed as an interaction on the Australian label, and no study was found showing it blocks absorption. Many vitamin D products also contain calcium, though, and those need the same several-hour gap as calcium.

What if I take levothyroxine at night?

Take it at least three hours after your last meal, on an empty stomach, as the American guideline describes. Keep iron, calcium, antacids and other mineral products earlier in the day, several hours away from it. Talk to your doctor before changing from morning to night dosing.

How long after levothyroxine can I have a protein shake?

A plain protein shake is food, so at least 30 minutes, ideally 60, after the tablet. If it has added minerals or soy protein, keep it several hours away, like a mineral supplement.

The honest takeaway

Levothyroxine timing comes down to two rules that are easy to blur: food needs 30 to 60 minutes, and iron, calcium and antacids need about 4 hours. Magnesium is kept several hours away. The exact numbers rest on thinner evidence than most people assume, but the direction is consistent. Pick a routine you can keep, space your supplements, and tell your doctor or pharmacist when anything you take changes.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

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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.
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Australian Commission on Safety and Quality in Health Care, Medicine Finder. Eutroxsig (levothyroxine) Consumer Medicine Information (May 2025) and Product Information (July 2025). safetyandquality.gov.au.