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Medicines and supplements

Vitamin C 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. NIH says vitamin C taken at the same time as levothyroxine might increase how much of the medicine is absorbed. A 2021 systematic review found the evidence limited to small studies, and the label says levothyroxine has a narrow therapeutic index, so changes in absorption matter.1,2,3

Key facts

  • NIH says vitamin C supplementation might increase the absorption of levothyroxine when taken at the same time.1
  • A 2014 study of 31 adults with an underactive thyroid and stomach conditions found lower TSH in every person while they swallowed levothyroxine with vitamin C, compared with their own earlier periods.4
  • A 2021 systematic review says the vitamin C evidence comes from an uncontrolled study and a non-randomized crossover study and needs to be proven in larger groups.2
  • The levothyroxine label says it has a narrow therapeutic index, so too much or too little can have negative effects.3
  • This documented effect points the other way from iron, calcium and fiber, which the label says can lower absorption.1,3

Ask your pharmacist or doctor first

Vitamin C taken at the same time as levothyroxine might increase how much of the medicine is absorbed.1

Reported in studies

Does vitamin C change how levothyroxine is absorbed?

MedlinePlus describes levothyroxine as a hormone that replaces the thyroid hormone the body would normally make. NIH's fact sheet on dietary supplements for immune function says vitamin C supplementation might increase the absorption of levothyroxine when the two are taken at the same time.1,5

A 2017 review of what affects levothyroxine absorption says vitamin C has the ability to raise it, while soybeans and coffee have the greatest effect in lowering it. The 2021 systematic review describes the proposed reasoning: vitamin C is known to lower stomach pH, and the review links a higher stomach pH with lower levothyroxine absorption.2,6

What did the vitamin C studies measure?

A 2014 study followed 31 adults with an underactive thyroid and stomach conditions whose thyroid tests were not well controlled. Each person was compared with their own earlier periods: two control periods of 2 months, then 2 months in which the levothyroxine dose was swallowed in water containing 500 mg of vitamin C. TSH fell in all 31 people, free T4 was higher in 30, and TSH reached the normal range in 17.4

The 2021 review describes a second, smaller study of 28 people whose TSH stayed high despite treatment. They took 1 g of vitamin C with their levothyroxine for 6 to 8 weeks, and their average TSH fell from about 9.0 to 2.3 mU/L.2

Neither study had a randomized comparison group, and the people studied had stomach problems or thyroid tests that were already off, so the results may not apply to everyone. This page describes what the studies measured. It does not suggest taking vitamin C with the medicine.

Why does a change in absorption matter with levothyroxine?

The label says levothyroxine has a narrow therapeutic index, so overtreatment or undertreatment may have negative effects, and that the right dose depends on factors including other medicines and the food taken with it. The label also tells prescribers to follow thyroid blood tests regularly. A 2017 review notes that because the medicine is taken in microgram amounts, absorption interactions can be clinically significant.3,6

The sources do not say what happens to levothyroxine absorption if someone who has been taking vitamin C stops it, or how long any effect lasts.

Where does vitamin C turn up besides a vitamin C supplement?

NIH says most multivitamins contain vitamin C, and that it is also sold alone and combined with other nutrients. NIH's statement about levothyroxine is about vitamin C supplementation. The sources checked do not address vitamin C from food and drink, which is not a finding either way.1,7

What to ask your pharmacist about vitamin C and levothyroxine

  • Does my thyroid dose assume I take vitamin C at the same time, or not?
  • If I start or stop a vitamin C product, should my thyroid blood tests be rechecked?
  • Does my multivitamin or immune blend contain vitamin C?
  • Do I take iron, calcium, fiber or antacids, which the label says can lower absorption?3

Questions people ask

Can you take vitamin C with levothyroxine?

NIH says vitamin C taken at the same time might increase levothyroxine absorption, and the evidence is limited to small studies. Because the medicine has a narrow therapeutic index, ask your pharmacist or doctor before combining them or changing how you take them.1,2,3

Should you take vitamin C with levothyroxine?

The sources checked do not recommend it. The 2021 review says the effect needs to be shown in larger studies. Whether it makes sense for you is for your prescriber to decide, with your thyroid tests in hand.2

Do vitamin C and levothyroxine interact?

NIH lists a possible interaction based on studies, in which vitamin C might raise absorption. That differs from iron, calcium and fiber, which the label says can lower it.1,3

Does vitamin C affect levothyroxine absorption?

NIH says it might raise it. In the 2014 study of 31 adults, TSH fell in every person while levothyroxine was swallowed with vitamin C, but there was no randomized comparison group.1,4

Can you take vitamin C with thyroid medication?

The sources on this page address levothyroxine. They do not cover other thyroid medicines, so ask your pharmacist about yours.3

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Sources

  1. National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Health Professionals. Updated March 10, 2025. Opened 2026-10-07.
  2. Wiesner A, Gajewska D, Pasko P. Levothyroxine Interactions with Food and Dietary Supplements: A Systematic Review. Pharmaceuticals (Basel). 2021;14(3):206. Opened 2026-10-11.
  3. Levothyroxine sodium tablets: prescribing information (Cardinal Health 107, LLC). DailyMed, U.S. National Library of Medicine. Version effective September 18, 2026. Opened 2026-10-11.
  4. Jubiz W, Ramirez M. Effect of vitamin C on the absorption of levothyroxine in patients with hypothyroidism and gastritis. J Clin Endocrinol Metab. 2014;99(6):E1031-E1034. Opened 2026-10-11.
  5. MedlinePlus. Levothyroxine. AHFS Patient Medication Information, American Society of Health-System Pharmacists. Last revised February 15, 2019. Opened 2026-10-11.
  6. Skelin M, Lucijanic T, Amidzic Klaric D, et al. Factors Affecting Gastrointestinal Absorption of Levothyroxine: A Review. Clin Ther. 2017;39(2):378-403. Opened 2026-10-11.
  7. Office of Dietary Supplements, National Institutes of Health. Vitamin C: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.

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