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Medicines and supplements

Multivitamins and levothyroxine: what to know

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. The levothyroxine label tells patients that iron and calcium supplements can decrease its absorption, and many multivitamins contain calcium, some with iron too. Biotin, found in some multivitamins, can also distort the thyroid blood tests your doctor uses to adjust the dose.1,2,3

Key facts

  • The levothyroxine label says calcium carbonate and ferrous sulfate may bind levothyroxine.1
  • NIH says levothyroxine is not absorbed well when taken too close to a calcium carbonate supplement.2
  • MedlinePlus lists niacin among the nonprescription products that may interact with levothyroxine.4
  • The label warns that biotin supplements can cause wrong thyroid test results.1

Ask your pharmacist how far apart to take them

Calcium and iron can reduce how much levothyroxine is absorbed. The levothyroxine label tells patients that iron and calcium supplements can decrease its absorption, and MedlinePlus names calcium carbonate and iron supplements, and also niacin, among products that may interact with it. Many multivitamins contain calcium, and some contain iron.1,2,4,5

On the medicine's label

Calcium and iron

The warning is on the medicine's own label. Its drug interactions section lists calcium carbonate and ferrous sulfate among agents that may bind levothyroxine, and its advice for patients says iron and calcium supplements and antacids can decrease absorption.1

NIH's fact sheets agree. The calcium sheet says levothyroxine is not absorbed well when taken too close to a calcium carbonate supplement, and the iron sheet says iron taken at the same time can reduce how well levothyroxine works.2,5

MedlinePlus names the same two, calcium carbonate and iron supplements, and tells people to keep them apart from their levothyroxine.4

Niacin

MedlinePlus also lists niacin among the nonprescription products that may interact with levothyroxine, and asks people not to start any of them without talking to their healthcare provider. Niacin is found in multivitamin and mineral products. The MedlinePlus page does not explain how the two interact.4,6

Biotin and thyroid tests

Some multivitamins contain biotin. The levothyroxine label warns that biotin supplements can interfere with the tests used to measure TSH and thyroid hormones and give wrong results, and it tells people to stop biotin before thyroid testing. NIH describes cases where biotin made thyroid tests look abnormal.1,3

A wrong result matters because the dose of levothyroxine is set by those tests. Tell your doctor or the lab that you take a multivitamin with biotin, and ask how long before a test to stop it.1

Check your multivitamin

  • Calcium, which may be listed as calcium carbonate2
  • Iron, which many multis made for women contain5
  • Niacin4
  • Biotin3

What to ask your pharmacist

  • How far apart should I take levothyroxine and my multivitamin?
  • Do the calcium and iron in my multivitamin matter at the amounts it contains?
  • Should I stop my multivitamin before my next thyroid blood test, and for how long?
  • Do my other supplements or antacids contain calcium, iron or biotin?

Questions people ask

Can I take levothyroxine and a multivitamin in the morning?

Not without asking first. The label tells patients to keep iron and calcium supplements apart from levothyroxine, and your pharmacist can set a schedule that fits both.1

Does the amount of calcium in my multivitamin matter?

The label and NIH name calcium carbonate and calcium supplements without giving an amount below which the warning stops applying, and this page does not give one either. Ask your pharmacist about your exact product.1,2

Why would a multivitamin change my thyroid test?

Because of biotin. Tests that use biotin in their chemistry can read falsely high or falsely low when you take biotin supplements, which could lead to the wrong dose.1,3

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Sources

  1. Levothyroxine sodium tablets: prescribing information (Cardinal Health 107). DailyMed, U.S. National Library of Medicine. Published September 24, 2026. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Consumers. Updated September 14, 2023. Opened 2026-10-07.
  3. National Institutes of Health, Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. Updated January 10, 2022. Opened 2026-10-07.
  4. MedlinePlus. Levothyroxine. U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
  5. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
  6. National Institutes of Health, Office of Dietary Supplements. Niacin: Fact Sheet for Health Professionals. Updated November 18, 2022. Opened 2026-10-01.

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