Levothyroxine works because its dose is matched to you: a prescriber adjusts it until blood tests put your thyroid-stimulating hormone (TSH) in a target range. That makes the question about ashwagandha a real one, because the herb appears to push thyroid hormone levels up on its own. Here is what the research shows, what it doesn't, and how to raise it with the person who manages your prescription.

This article is educational and is not medical advice. Don't start, stop or change levothyroxine or any supplement based on a web article. If you take thyroid medication, talk to your prescriber or pharmacist before adding ashwagandha.

What the research shows about ashwagandha and thyroid hormones

The clearest trial gave 50 adults with subclinical hypothyroidism (a raised TSH of 4.5 to 10 without obvious symptoms) either 600 mg of ashwagandha root extract a day or a placebo for eight weeks. Compared with placebo, TSH, T3 and T4 all moved toward the normal range (Sharma 2018). It was a small, single-centre pilot study, and eight weeks is short.

A second signal came by accident. In an eight-week trial of ashwagandha in people with bipolar disorder, thyroid levels were measured only for safety. All three ashwagandha-treated patients with abnormal thyroid values saw T4 rise, by 7% to 24%, while most of the placebo group's T4 fell (Gannon 2014). The numbers are tiny, and the authors' own conclusion was that vigilance about an overactive thyroid may be warranted.

Across 23 randomized trials with 1,706 participants, a 2026 meta-analysis found no average effect on TSH or T3 but a modest rise in T4 (Fornalik 2026). On average, then, the effect is small. The case reports below show it can be larger in some people.

When it goes too far: case reports

Several case reports describe thyrotoxicosis, meaning too much thyroid hormone, after ashwagandha. A 73-year-old woman who had taken it for two years to treat her own underactive thyroid arrived with a fast, abnormal heart rhythm (supraventricular tachycardia) and a very low TSH. In hospital her rhythm was brought back to normal with emergency heart medication, she was started on a beta-blocker and she was told to stop ashwagandha. Two weeks later her symptoms had resolved and her TSH was back in the normal range (Kamal 2022). A previously healthy 47-year-old man developed painless thyroiditis two months after starting it, and recovered after stopping (Hayashi 2024). An earlier Dutch report described the same pattern (van der Hooft 2005). Case reports can't tell you how often this happens. They show that it can.

What nobody has studied: ashwagandha with levothyroxine

We found no trial that tested ashwagandha in people taking levothyroxine (PubMed search, October 2026). Nobody knows how often adding it pushes a well-adjusted dose into over-replacement, or whether it changes how levothyroxine is absorbed. The concern is straightforward, though: if your dose was set to land your TSH in range, anything that raises thyroid hormone on top of it could take you past that range. The U.S. National Center for Complementary and Integrative Health lists thyroid hormone medications among the drugs ashwagandha might interact with, and says it is not recommended for people with thyroid disorders, including autoimmune ones such as Hashimoto's.

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What to ask your prescriber

  • Tell whoever manages your levothyroxine before you start, and bring the product and its dose.
  • Ask whether your thyroid levels should be rechecked after you start or stop it. Your levothyroxine dose was set without it.
  • Know the signs of too much thyroid hormone: a racing or irregular heartbeat, tremor, feeling hot, unexplained weight loss, trouble sleeping or new anxiety. If you notice them, contact your clinician.
  • Don't use ashwagandha to treat an underactive thyroid yourself. The one positive trial was in mild, untreated cases under study conditions, and the most serious case report involved exactly this kind of self-treatment.

The bottom line

Ashwagandha is not a neutral herb for someone on thyroid medication. The trials suggest a modest upward push on thyroid hormone, the case reports show it can occasionally overshoot, and no study has tested it alongside levothyroxine. That doesn't mean it's off-limits for everyone, but it does mean the decision belongs with the person who manages your thyroid dose, ideally with a plan to recheck your levels.

References

  • Sharma AK, Basu I, Singh S. "Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: a double-blind, randomized placebo-controlled trial." Journal of Alternative and Complementary Medicine, 2018;24(3):243–248. PMID: 28829155.
  • Gannon JM, Forrest PE, Roy Chengappa KN. "Subtle changes in thyroid indices during a placebo-controlled study of an extract of Withania somnifera in persons with bipolar disorder." Journal of Ayurveda and Integrative Medicine, 2014;5(4):241–245. PMID: 25624699.
  • Fornalik M, Malkiewicz A, Adamczak D, et al. "Hormonal modulation with Withania somnifera: systematic review and meta-analysis of randomized-controlled trials." Planta Medica, 2026;92(8):790–805. PMID: 41740946.
  • Kamal HI, Patel K, Brdak A, et al. "Ashwagandha as a unique cause of thyrotoxicosis presenting with supraventricular tachycardia." Cureus, 2022;14(3):e23494. PMID: 35475098.
  • Hayashi M, Hamada H, Azuma SI, Hayashi K. "Painless thyroiditis by Withania somnifera (ashwagandha)." Cureus, 2024;16(3):e55352. PMID: 38559552.
  • van der Hooft CS, Hoekstra A, Winter A, et al. "Thyrotoxicosis following the use of ashwagandha." Nederlands Tijdschrift voor Geneeskunde, 2005;149(47):2637–2638. PMID: 16355578.
  • National Center for Complementary and Integrative Health. "Ashwagandha." Fact sheet, last updated March 2023. nccih.nih.gov/health/ashwagandha