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Ashwagandha and levothyroxine: what to know before combining them

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 ashwagandha might interact with thyroid hormone medicines such as levothyroxine. In a trial, 600 mg a day of ashwagandha raised T3 and T4 and lowered TSH within 8 weeks, and it has been linked to cases of an overactive thyroid.1,2,3

Key facts

  • Levothyroxine is synthetic T4, chemically identical to the hormone the thyroid gland makes.4
  • The levothyroxine label says it has a narrow therapeutic index, and that the dose is judged by periodic lab tests.4
  • In three case reports, an overactive thyroid in women taking ashwagandha resolved after they stopped it.1
  • NIH's complementary health center does not recommend ashwagandha for people with thyroid disorders.5

Ask your pharmacist or doctor first

NIH says ashwagandha might interact with thyroid hormone medicines, such as levothyroxine. The concern comes from studies in which ashwagandha raised thyroid hormone levels, and from case reports of an overactive thyroid in people taking it.1,2,5,6

Expected from how the two work, not shown in studies

What is documented

NIH's consumer fact sheet says a few studies show ashwagandha may affect how the thyroid gland works, and that it therefore might interact with thyroid medications. NIH's complementary health center lists thyroid hormone medications among the medicines ashwagandha might interact with, and does not recommend it for people with thyroid disorders.2,5

The sources on this page do not describe a study of ashwagandha taken together with levothyroxine. The concern follows from how the two work: levothyroxine supplies T4, and ashwagandha has raised T4 in studies.1,4

What ashwagandha did to thyroid levels

In a trial of 50 adults with a mildly underactive thyroid, 600 mg a day of root extract for 8 weeks lowered TSH and raised T3 and T4 compared with placebo. In a smaller study, three men taking 500 mg a day of a root and leaf extract for 8 weeks had small rises in T4.1,3

Three case reports describe an overactive thyroid in women aged 32 to 73 who were taking ashwagandha. One took an unusually high 1,950 mg a day for more than 2 months. In each case, stopping ashwagandha resolved the problem.1

Not every study found a change. In an 8-week safety trial of 80 healthy adults, thyroid hormone levels did not change significantly, but that trial enrolled healthy volunteers, not people taking thyroid medicine.7

Why small changes matter with levothyroxine

The levothyroxine label calls it a medicine with a narrow therapeutic index: too much or too little can cause problems, so the dose is set and checked with lab tests. Its side effects are mainly those of too much thyroid hormone, such as a fast or irregular heartbeat, tremor, nervousness and trouble sleeping.4

MedlinePlus tells people taking levothyroxine to tell their doctor and pharmacist about every vitamin, supplement and herbal product they take or plan to take, because doses may need to change. It also says to keep all appointments for lab tests.6

What to ask your pharmacist or doctor

  • Should I take ashwagandha at all, given my thyroid condition?
  • If I start or stop ashwagandha, should my thyroid levels be checked sooner than usual?
  • Which signs of too much thyroid hormone should make me call you?
  • Do any of my other supplements contain ashwagandha?

Bring the Supplement Facts panel of everything you take. Ashwagandha also appears in blends, sometimes under other names such as winter cherry or Indian ginseng.1

Questions people ask

Can ashwagandha replace thyroid medicine?

No. Nothing in these sources suggests that ashwagandha can stand in for thyroid hormone. MedlinePlus says not to stop taking levothyroxine without talking to your doctor.6

What if I have an underactive thyroid but take no medicine?

NIH's complementary health center does not recommend ashwagandha for people with thyroid disorders, whether or not they take medicine. Ask your doctor before starting it.5

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Sources

  1. National Institutes of Health, Office of Dietary Supplements. Ashwagandha: Is it helpful for stress, anxiety, or sleep? Fact Sheet for Health Professionals. Updated May 2, 2025. Opened 2026-10-07.
  2. Office of Dietary Supplements, National Institutes of Health. Ashwagandha: Is it helpful for stress, anxiety, or sleep? Fact Sheet for Consumers. Updated December 4, 2023. Opened 2026-10-07.
  3. Sharma AK, Basu I, Singh S. Efficacy and Safety of Ashwagandha Root Extract in Subclinical Hypothyroid Patients: A Double-Blind, Randomized Placebo-Controlled Trial. J Altern Complement Med. 2018;24(3):243-248. Opened 2026-10-07.
  4. Levothyroxine sodium tablets: prescribing information (Alvogen). DailyMed, U.S. National Library of Medicine. Published September 17, 2026. Opened 2026-10-07.
  5. National Center for Complementary and Integrative Health. Ashwagandha. Last updated March 2023. Opened 2026-10-07.
  6. MedlinePlus. Levothyroxine. U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
  7. Verma N, Gupta SK, Tiwari S, Mishra AK. Safety of Ashwagandha Root Extract: A Randomized, Placebo-Controlled, study in Healthy Volunteers. Complement Ther Med. 2021;57:102642. Opened 2026-10-07.

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