Can I Take Ashwagandha With Levothyroxine? What the Trials Show

Not without your prescriber. Pooled across 23 randomised trials in 1,706 people, ashwagandha root extract raised thyroxine (T4) by 0.61 µg/dL while TSH and T3 did not change; in 50 people with subclinical hypothyroidism, 600 mg a day for 8 weeks lowered TSH by about 17% and raised T4 by about 20% against placebo; and three published case reports describe thyrotoxicosis that resolved when ashwagandha was stopped.^{1, 2, 3, 4, 5} The direction is one way, up. So for anyone whose thyroid hormone level is set by a tablet, levothyroxine or liothyronine, or whose thyroid is overactive, autoimmune, treated with radioiodine or removed, ashwagandha is a conversation with the GP or endocrinologist before the first capsule, and it is never a replacement for thyroid medicine. Part of the Wellgard ashwagandha guide.

What does ashwagandha do to thyroid hormones?

It raises thyroxine. The 2026 meta-analysis pooled 23 placebo-controlled trials of root extract, 1,706 participants, and found T4 up 0.61 µg/dL (p=0.02) with no significant change in TSH or T3.¹ The clearest single trial was in people whose thyroid was already struggling: 50 adults with untreated subclinical hypothyroidism took 600 mg a day or placebo for 8 weeks, and on ashwagandha TSH fell about 17.5% and T4 rose about 19.6%.² A safety sub-analysis of a US trial in bipolar disorder found that all three treated patients whose thyroid indices moved showed T4 rises, of 7%, 12% and 24%, while six of seven on placebo fell.³ How it does this is not established: stimulation of the gland and faster conversion of T4 to T3 have both been proposed. What the data rule out is the forums' favourite reassurance, an "adaptogen" that "balances either way". In every dataset the hormone moved one way.

Can you take ashwagandha with levothyroxine?

Levothyroxine is thyroxine. Your dose is set by blood test to hold your T4 and TSH in range, and the NHS describes it as a lifelong medicine with regular tests because the wrong dose causes side effects.⁶ Add something that raised T4 by a fifth, and a dose that was right can become too much. Too much thyroid hormone feels like the overactive-thyroid list: palpitations, tremor, heat intolerance, nervousness, difficulty sleeping, weight loss, loose bowels.⁶ The UK thyroid-forum member whose free T3 "shot up" with tremor and anxiety 12 days into 300 mg a day described it exactly. This is not an absorption interaction. Iron, calcium and coffee bind levothyroxine in the gut, which is why the tablet is taken alone on an empty stomach;⁶ ashwagandha acts on the hormone levels themselves, so "take them hours apart" solves nothing. If you take levothyroxine and want to try it, the order is: tell the prescriber, agree a thyroid function test 6 to 8 weeks in, know the symptoms above, and stop and ring the surgery if they appear.

What about liothyronine, Hashimoto's or Graves' disease?

Liothyronine is T3. The pooled trials found T3 unchanged, but the case above was a free T3 rise on combined treatment, and every argument for levothyroxine applies: the level is set by a tablet and ashwagandha pushes the system up. Hashimoto's thyroiditis, the usual cause of an underactive thyroid in the UK, adds a second caution: it is autoimmune, and ashwagandha has immune-stimulating activity in laboratory work, which is why the US National Institutes of Health's complementary-health institute advises people with autoimmune disease against it. Nothing about Hashimoto's makes a rise in T4 safer. Graves' disease and any other overactive thyroid are the clearest no: the gland is already making too much hormone, carbimazole or propylthiouracil is holding it down, and adding something that raises T4 is the wrong direction entirely. Two of the three thyrotoxicosis case reports were in people with no known thyroid disease.^{4, 5}

What if you have had radioiodine, or your thyroid removed?

"It can't affect a thyroid you haven't got" is the post-radioiodine thread's conclusion, and it is the wrong way round. After radioiodine or a thyroidectomy your thyroxine comes entirely from the tablet, so there is no gland to stimulate; but whether ashwagandha's effect runs through the gland, through conversion of T4 to T3 in the liver and tissues, or through how fast the hormone is cleared is not known, and no trial has included people without a thyroid. Unknown is not safe. The member who tried it after radioiodine and had three good days, then dizziness, bloating and "hyper and hypo" symptoms, was describing an unstable level. If tiredness on a stable dose is the problem, a full panel, TSH, free T4 and free T3, plus ferritin, B12, folate and vitamin D, comes first, because under-replacement and iron deficiency are commoner than anything a supplement would change.

What do the thyrotoxicosis case reports show?

Three, with the same shape. In 2005 a healthy 32-year-old Dutch woman developed thyrotoxicosis after increasing her ashwagandha dose, and it resolved once she stopped: the first human report.⁵ In 2022 a 73-year-old woman who had taken ashwagandha for two years to self-manage an underactive thyroid presented with a fast heart rhythm (supraventricular tachycardia) from thyrotoxicosis, which settled when she stopped.⁴ In 2024 a healthy 47-year-old Japanese man developed painless thyroiditis with thyrotoxicosis after two months, again resolving on withdrawal.⁷ Three cases is not a rate, and the 2026 safety review of standardised root extract, 23 trials in 2,317 healthy adults, found endocrine markers inside normal ranges for up to 180 days.⁸ But those trials screened out people with thyroid disease or on thyroid medicine, and the 73-year-old is the cautionary one: self-managing a thyroid condition with ashwagandha is how a modest rise becomes an overshoot nobody is measuring.

What would a prescriber monitor if they agreed?

The same tests they use for your dose. Thyroid function, TSH and free T4, with free T3 if you take liothyronine, about 6 to 8 weeks after starting, then at the usual interval; your pulse and the symptom list; and a plan to stop if TSH falls below range or symptoms of excess appear.⁶ The trials ran 8 weeks; nobody has measured what a year of ashwagandha does to a dose stable for a decade. Take the pack to the appointment so the extract and its milligrams go in the notes, stay at the label dose, and do not adjust your levothyroxine yourself in either direction. A suspected reaction can be reported through the MHRA Yellow Card scheme, which covers supplements as well as medicines.⁹

Do you need to test your cortisol first?

No. The thyroid forums treat a private saliva or urine cortisol test as a prerequisite, on the logic that ashwagandha "lowers cortisol" and will lower a low level further. The trial basis for the first half is one 60-day study of 64 chronically stressed adults in which 600 mg a day lowered serum cortisol by about 28% against 8% on placebo; pooled trials point the same way with low certainty, and a 2023 systematic review concluded that no study has asked what a lower cortisol means clinically.^{10, 11} The second half is an assumption: no trial has given ashwagandha to people with low cortisol, and a persistently low morning cortisol is a medical finding, adrenal insufficiency, for an endocrinologist rather than a supplement decision. A high result is equally a GP conversation. If sleep is why you were considering ashwagandha, which sleep supplements have evidence behind them sets its 5-minute result beside the alternatives, and why melatonin is prescription-only in the UK explains the medicine route.

What if you are already taking both?

Do not stop the levothyroxine. Stop the ashwagandha if you have any symptom from the overactive list, and tell the GP either way, with the extract and its dose, so they can decide whether to bring a blood test forward. What matters is that the next result is read knowing the supplement is there. Who should not take ashwagandha at all, the liver cases and the full medicine table are on who should not take ashwagandha, and how long people take it and what happens on stopping are on is ashwagandha safe to take daily.

Frequently asked questions

Does ashwagandha affect TSH levels?

In the pooled trials TSH did not change significantly while T4 rose; in the one trial in subclinical hypothyroidism TSH fell about 17% in 8 weeks. On levothyroxine, a falling TSH means too much hormone, not a healthier thyroid.^{1, 2}

Can ashwagandha replace levothyroxine, or let me lower my dose?

No. A rise in T4 in people with a working gland is not a replacement hormone, no trial has tested ashwagandha against or alongside levothyroxine, and dose changes are made on blood tests by the prescriber.²

Is ashwagandha good for an underactive thyroid?

One 8-week trial in 50 people with mild, untreated subclinical hypothyroidism improved TSH and T4; that is a research finding, not a treatment, and a diagnosed underactive thyroid is managed by the GP with levothyroxine.^{2, 6}

Can I take ashwagandha if I have Hashimoto's?

Only with your prescriber's agreement: the T4 rise applies to you, and the autoimmune caution adds to it.

Does ashwagandha interfere with thyroid blood tests?

Not with the assay; it changes the hormone levels themselves. Biotin in some supplement blends can distort thyroid immunoassays, so tell whoever takes the blood about every supplement.

Sources

1. Fornalik M, et al. Hormonal modulation with Withania somnifera: systematic review and meta-analysis of randomized-controlled trials. Planta Medica, 2026. PubMed 41740946.
2. 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. PubMed 28829155.
3. 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. PubMed 25624699.
4. Kamal HI, et al. Ashwagandha as a unique cause of thyrotoxicosis presenting with supraventricular tachycardia. Cureus, 2022. PubMed 35475098.
5. van der Hooft CS, et al. Thyrotoxicosis following the use of ashwagandha. Nederlands Tijdschrift voor Geneeskunde, 2005. PubMed 16355578.
6. NHS. Levothyroxine. nhs.uk/medicines/levothyroxine; NHS. Overactive thyroid (hyperthyroidism). nhs.uk.
7. Hayashi M, et al. Painless thyroiditis by Withania somnifera (ashwagandha). Cureus, 2024. PubMed 38559552.
8. Coope OC, et al. Back to the roots: safety and tolerability of standardised ashwagandha (Withania somnifera) root extract in healthy adults. Pharmaceuticals, 2026. PubMed 42198398.
9. Medicines and Healthcare products Regulatory Agency. Yellow Card scheme. yellowcard.mhra.gov.uk.
10. Chandrasekhar K, Kapoor J, Anishetty S. A prospective, randomized double-blind, placebo-controlled study of safety and efficacy of a high-concentration full-spectrum extract of ashwagandha root in reducing stress and anxiety in adults. Indian Journal of Psychological Medicine, 2012. PubMed 23439798.
11. Della Porta M, Maier JA, Cazzola R. Effects of Withania somnifera on cortisol levels in stressed human subjects: a systematic review. Nutrients, 2023. PubMed 38140274.