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Medicines and supplements

Calcium 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. Calcium supplements and calcium antacids can attach to levothyroxine in the stomach and cut the amount that gets absorbed. In a study of 20 people, taking calcium carbonate with levothyroxine for 3 months raised TSH. NIH and the drug label both say to keep them apart.1,2,3

Key facts

  • NIH says levothyroxine is not absorbed well when taken close to a calcium carbonate supplement.1
  • In 20 people on long-term levothyroxine, 1,200 mg of calcium a day taken with it for 3 months raised average TSH from 1.6 to 2.7 mIU/L, and 1 in 5 went above the normal range.2
  • In a small study, calcium carbonate, citrate and acetate each cut levothyroxine absorption by about 20% to 25%.4
  • MedlinePlus advises keeping calcium carbonate apart from other medicines because it can make them less effective.5

Ask your pharmacist how far apart to take them

Calcium carbonate can bind levothyroxine in the gut and reduce how much is absorbed. The levothyroxine label lists calcium carbonate among products to keep apart from the medicine, and a small study found calcium citrate and acetate had a similar effect; ask your pharmacist how to space them.1,3,4,6,7

On the medicine's label

What NIH and the label say

NIH's fact sheet for health professionals says calcium carbonate supplements can interfere with the absorption of levothyroxine, a thyroid hormone used for an underactive thyroid and for thyroid cancer, and that the FDA-approved label instructs patients taking calcium carbonate to keep it apart from their levothyroxine.6

The label itself puts calcium carbonate alongside ferrous sulfate and other agents that may stick to levothyroxine, and its counseling section warns that calcium and iron supplements and antacids can reduce absorption. MedlinePlus repeats the instruction for calcium carbonate in its patient information.3,7

Carbonate, citrate and antacids

The warnings name calcium carbonate, the form in many supplements and in antacids. A small study suggests the other common forms behave the same way: in 8 healthy adults, calcium carbonate, citrate and acetate each lowered levothyroxine absorption by roughly a fifth to a quarter.4,6

Calcium also turns up in multivitamins, in calcium and vitamin D combinations and in antacid chews, so a label check matters even if you do not think of yourself as taking a calcium supplement.6

What the studies measured

In a 2000 study, 20 adults who had taken levothyroxine for years added 1,200 mg of calcium a day as calcium carbonate, swallowed together with their levothyroxine, for 3 months. Free and total T4 fell, average TSH rose from 1.6 to 2.7 mIU/L, and levels went back to where they started once the calcium was stopped. In the test tube, levothyroxine stuck to calcium carbonate in acid, as it would in the stomach.2

Both studies were small, and the single-dose study followed absorption for a short time after one dose. They show why spacing matters, not what gap is enough; that is a question for your pharmacist.2,4

What to ask your pharmacist

  • Which of my products contain calcium, including antacids and multivitamins?
  • How should I time my calcium around my levothyroxine?
  • Does the form of calcium I take change the advice?
  • Should my TSH be checked after I start or stop calcium?

If you take a thyroid medicine other than levothyroxine, the sources on this page do not cover it, so ask your pharmacist about it specifically.

Questions people ask

Does calcium citrate interact with levothyroxine too?

In a small study of 8 healthy adults, calcium citrate lowered levothyroxine absorption about as much as calcium carbonate did. Ask your pharmacist how to time any form of calcium.4

Do calcium antacids count?

Yes. The levothyroxine label names antacids, and many antacids are made of calcium carbonate.3,6

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Sources

  1. Office of Dietary Supplements, National Institutes of Health. Calcium: Fact Sheet for Consumers. Updated September 14, 2023. Opened 2026-10-07.
  2. Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283(21):2822-2825. Opened 2026-10-07.
  3. Levothyroxine sodium tablets: prescribing information (Alvogen). DailyMed, U.S. National Library of Medicine. Effective September 15, 2026. Opened 2026-10-07.
  4. Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid. 2011;21(5):483-486. Opened 2026-10-07.
  5. MedlinePlus, U.S. National Library of Medicine. Calcium Carbonate: MedlinePlus Drug Information. Last revised June 20, 2024. Opened 2026-10-07.
  6. Office of Dietary Supplements, National Institutes of Health. Calcium: Fact Sheet for Health Professionals. Updated June 22, 2026. Opened 2026-10-07.
  7. MedlinePlus, U.S. National Library of Medicine. Levothyroxine: MedlinePlus Drug Information. Last revised February 15, 2019. Opened 2026-10-07.

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