How Far Apart Should I Take Iron and Levothyroxine?

Take iron supplements at least 4 hours apart from levothyroxine. The levothyroxine prescribing information says so, and the NIH Office of Dietary Supplements repeats it. Taken together, iron binds to the hormone in your gut, and less of your dose reaches your blood.

How strong is the evidence? Moderate that taking them together is a problem. It is in the prescribing information, the NIH iron fact sheet and the American Thyroid Association guideline, and a small clinical trial showed it. The studies are small, though: the ATA grades its own separation advice as weak, on weak-quality evidence, and calls the 4-hour figure "traditional, but untested". Nobody has shown that a shorter gap is enough, so 4 hours is the gap the label asks for.

What happens when you take them together

In a study from Canada, 14 people with hypothyroidism on a steady levothyroxine dose took an iron tablet (ferrous sulfate) together with their thyroid pill every day for 12 weeks (Campbell 1992).

• Their average TSH, the blood test used to judge the dose, rose to more than three times where it started. (Here is what TSH measures.)
• About two in three had more signs and symptoms of an underactive thyroid.
• When the researchers mixed iron and levothyroxine in a test tube, they formed a complex that barely dissolved. The iron was binding the hormone.

The effect varied from person to person, but for some it was big enough to matter. The study was small and had no comparison group, yet it matches how the label and the NIH describe the interaction.

One morning when they overlap is not an emergency. Levothyroxine stays in the body a long time, with a half-life of about a week (ATA guideline). The problem in the study was a daily habit over weeks. If it happens, your pharmacist can tell you whether to do anything about it, and mention it to your doctor if it keeps happening.

Iron is fussy too

Iron has its own needs. NHS advice for ferrous sulfate, a common iron tablet, says:

• It works best on an empty stomach: 30 minutes before eating or 2 hours after.
• Don't take it with tea, coffee, eggs or dairy, and leave a 2-hour gap after it before having them, because they reduce how much iron gets in.
• If it upsets your stomach, it can be taken with or after food.

Calcium might also reduce how much iron you absorb, so the NIH fact sheet says experts suggest taking calcium and iron supplements at different times of day. And calcium supplements need the same 4-hour gap from levothyroxine as iron (prescribing information).

So the puzzle is: thyroid pill alone on an empty stomach, with a 30 to 60-minute wait before food; iron at least 4 hours away from it; and iron ideally away from meals, coffee, tea and dairy.

Two sample days that work

These are examples to show the shape of a day, not a prescription. Your doctor or pharmacist sets your times.

If you take levothyroxine in the morning

Time	What
6:30	Levothyroxine with water
7:30	Breakfast and coffee
12:30	Lunch
14:30	Iron with water
From 16:30	Tea, coffee and dairy again

If you take levothyroxine at bedtime

Time	What
7:30	Breakfast and coffee
10:00	Iron with water
From 12:00	Lunch, tea and coffee as usual
19:00	Dinner
22:30	Levothyroxine with water, 3 or more hours after dinner

If that feels fiddly, the simplest version is: thyroid pill first thing, iron in the early afternoon. Thyroid Coach's spacing assistant flags an iron reminder set within 4 hours of your levothyroxine time, and checks again if you log one pill too soon after the other.

Multivitamins count. If yours contains iron or calcium, give it the same 4-hour gap. The ATA guideline notes that multivitamins haven't been studied directly, but their iron and calcium would be expected to block absorption.

Every-other-day iron. If iron gives you side effects, the NHS says your doctor may suggest taking it on alternate days. In a trial in iron-depleted women, alternate-day dosing absorbed more iron per dose than daily dosing (Stoffel 2017; evidence: Moderate). That trial measured absorption over a few weeks, not recovery, and its authors said the finding still needs confirming in people with anaemia, so it is one option among several rather than a better way for everyone. It is a question for your doctor, not a change to make on your own. On iron days, the 4-hour gap still applies.

Why the timing is worth the effort

Iron deficiency anaemia can cause tiredness and lack of energy, along with shortness of breath and, less often, hair loss. Heavy periods and pregnancy are very common causes (NHS). If you are treating low iron and an underactive thyroid at the same time, you want both pills to work. Low iron is also one of the things worth checking if you still feel tired on levothyroxine.

Iron treatment takes a while: the NHS says iron tablets for anaemia are usually taken for about 6 months. A schedule you can keep up for that long matters more than a perfect one.

What to tell your doctor or pharmacist

• That you take both, and when. Include multivitamins, calcium and antacids. The full levothyroxine routine lists the usual ones to space out.
• If you have been taking them together or close together. Once you separate them, more of your levothyroxine may get in, so your doctor may want to recheck your TSH.
• When you start, finish or change your iron. Changing what sits next to your thyroid pill can change your levels.
• If iron upsets your stomach. Say so rather than quietly stopping. Taking it with food or on alternate days are options your doctor can weigh up.
• A question worth asking: "What times would you suggest for my iron and my thyroid pill?"

Your doctor or pharmacist knows your situation and has the final say on how you take your medicines.

Some safety points:

• Keep iron tablets out of reach of children. The NHS warns that an iron overdose in a young child can be fatal.
• If your periods are heavy and affecting your life, see your doctor (NHS).
• Chest pain, fainting or severe shortness of breath need urgent medical help.
• If you are pregnant or think you might be, contact your doctor as soon as you know. It is time-sensitive, because pregnancy can change how much levothyroxine you need (prescribing information).

Questions

Can iron and levothyroxine be taken together?

Not at the same time. The prescribing information says not to take levothyroxine within 4 hours of iron supplements, and in a small clinical trial, taking them together every day pushed TSH up. Take them at different times of day, at least 4 hours apart.

How many hours between iron and levothyroxine?

At least 4 hours, in either direction. The label says to take levothyroxine at least 4 hours before or after medicines that interfere with its absorption, and it names iron. A longer gap is fine.

Does the type of iron matter?

The label's warning covers iron supplements in general, not one brand or form. The main study used ferrous sulfate, and we found no good evidence that other forms, such as iron bisglycinate, slow-release or liquid iron, are safe to take with levothyroxine. Keep the 4-hour gap unless your pharmacist tells you otherwise.

Can I take a multivitamin with levothyroxine?

Not at the same time, if it contains iron or calcium. The ATA guideline says multivitamins haven't been studied directly, but their iron and calcium would be expected to block absorption. Give them the same 4-hour gap.

Sources

1. AbbVie. SYNTHROID (levothyroxine sodium) tablets, prescribing information, revised February 2024. DailyMed, US National Library of Medicine.
2. NIH Office of Dietary Supplements. Iron: fact sheet for health professionals. Updated 4 September 2025.
3. Jonklaas J, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751.
4. Campbell NR, et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992;117(12):1010–1013.
5. NHS. How and when to take ferrous sulfate. Page last reviewed 9 February 2023.
6. NHS. Iron deficiency anaemia. Page last reviewed 26 January 2024.
7. Stoffel NU, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women. Lancet Haematol. 2017;4(11):e524–e533.
8. NHS. Heavy periods. Page last reviewed 19 September 2024.