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Medicines and supplements

Iron supplements and levothyroxine

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Ask your pharmacist or doctor before combining. Iron supplements can bind levothyroxine in the gut and lower how much of it is absorbed, which can make the thyroid medicine less effective. NIH, the levothyroxine label and MedlinePlus all say to keep the two apart; your pharmacist can set the timing.1,2,3

Key facts

  • NIH says taking iron with levothyroxine can reduce the medicine's effectiveness.1
  • The levothyroxine label lists ferrous sulfate among products that may bind levothyroxine and keep it from being absorbed.2
  • MedlinePlus tells people who take ferrous sulfate to take it apart from their levothyroxine.3
  • In a 1992 study, 14 people who took a ferrous sulfate tablet with their thyroxine every day for 12 weeks saw their TSH rise from 1.6 to 5.4 mU/L.4

Ask your pharmacist how far apart to take them

Iron supplements can reduce how much levothyroxine the body absorbs and make it less effective. The levothyroxine label lists ferrous sulfate among products that may bind the medicine and says to take them apart; ask your pharmacist how to space them.1,2,3,5

On the medicine's label

What is documented

NIH's fact sheet for health professionals says that taking iron and levothyroxine at the same time can cause clinically significant reductions in how well levothyroxine works in some patients, and that the labels of some levothyroxine products warn about it.5

The current label groups ferrous sulfate with calcium carbonate and other products that may bind levothyroxine and stop it being absorbed. Its patient counseling section says iron and calcium supplements and antacids can decrease absorption, and it tells patients to keep these products apart from their tablet.2

The problem is binding in the gut: iron and thyroxine mixed in a test tube formed a poorly soluble complex. Less of the medicine then reaches the blood, and thyroid blood tests can drift as though the dose were too low.4

What a study measured

In the 1992 study, 14 adults on a stable dose of thyroxine for an underactive thyroid took a 300 mg ferrous sulfate tablet at the same moment as their thyroxine every day for 12 weeks. Their average TSH, the blood test used to adjust the dose, rose from 1.6 to 5.4 mU/L, and 9 of the 14 showed more signs of an underactive thyroid.4

It was a small study without a comparison group, and it tested taking both at once. It shows why the timing matters, not how far apart the two need to be.4

Where iron turns up

Iron is in iron-only supplements and in many multivitamins, especially those made for women; multivitamins for men and older adults often have less or none. Check the Supplement Facts panel of everything you take for ferrous sulfate, ferrous gluconate, ferrous fumarate or ferric salts.5

Bring those panels, or photos of them, to your pharmacist, so the advice covers every product rather than just the one labeled as iron.

What to ask your pharmacist

  • How far apart should I take levothyroxine and my iron supplement?
  • Does my multivitamin or prenatal vitamin contain iron or calcium?
  • Should my thyroid blood test be repeated after I start or stop taking iron?
  • Do my other medicines, such as antacids or acid reducers, affect my levothyroxine too?

The levothyroxine label also names calcium carbonate, antacids and proton pump inhibitors among products that can reduce its absorption, so the answer may cover more than iron.2

Questions people ask

Can I take iron and levothyroxine at the same time?

The sources on this page say not at the same time: NIH, the label and MedlinePlus all advise keeping them apart. Ask your pharmacist for a schedule that fits your other medicines.1,2,3

Does this apply to a multivitamin with iron?

A multivitamin with iron contains the same kind of iron salts, and the levothyroxine label also names calcium, which many multivitamins contain. Ask your pharmacist whether to space your multivitamin the same way.2,5

What about other thyroid medicines?

The sources on this page address levothyroxine. Ask your pharmacist about any other thyroid medicine you take.

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Sources

  1. Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Consumers. Updated August 17, 2023. Opened 2026-10-07.
  2. Levothyroxine sodium tablets: prescribing information (Alvogen). DailyMed, U.S. National Library of Medicine. Effective September 15, 2026. Opened 2026-10-07.
  3. MedlinePlus, U.S. National Library of Medicine. Levothyroxine: MedlinePlus Drug Information. Last revised February 15, 2019. Opened 2026-10-07.
  4. Campbell NR, Hasinoff BB, Stalts H, Rao B, Wong NC. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992;117(12):1010-1013. Opened 2026-10-07.
  5. Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.