Ashwagandha can raise thyroid hormone levels and stack additively with levothyroxine, risking thyrotoxicosis.
What's happening
Ashwagandha (Withania somnifera) stimulates thyroid output in humans at 500-600 mg/day over 8 weeks, raising T3 and T4 and lowering TSH. In a double-blind RCT of 50 subclinical hypothyroid patients (Sharma 2018), ashwagandha 600 mg/day significantly lowered TSH (p<0.001) and raised T3 (p=0.0031) and T4 (p=0.0096) versus placebo. In bipolar patients at 500 mg/day (Gannon 2014), free T4 rose by 7-24% in three treated patients, though the between-group difference was not statistically significant. The effect appears to depend on baseline thyroid state: a separate safety RCT in healthy adults at 300 mg twice daily found no significant thyroid change. No RCT has directly co-administered ashwagandha with levothyroxine. A 2022 case report (Cureus) documented thyrotoxicosis with supraventricular tachycardia in a 73-year-old woman after two years of daily ashwagandha; the authors flagged either glandular stimulation or possible exogenous T3/T4 contamination of the supplement. Memorial Sloan Kettering's monograph notes ashwagandha 'may increase thyroxine levels' and catalogs multiple thyrotoxicosis case reports. For a patient on stable levothyroxine, the plausible effect size (7-24% T4 rise) is large enough to move a well-titrated patient into a suppressed-TSH range. Timing-based workarounds do not help — the effect is endocrine over weeks, not absorption-based. Populations to watch: anyone on thyroid hormone replacement or antithyroid drugs, anyone with a history of thyroid disease even if off medication, and pregnancy/pediatrics (not studied, avoid).
Recommendation
If you take levothyroxine or any thyroid medication, consult your prescribing clinician before adding ashwagandha. If you proceed, retest TSH and free T4 within 4-8 weeks of starting, and again with any dose or brand change. Watch for palpitations, unintentional weight loss, heat intolerance, or anxiety; these suggest the combination has pushed you toward hyperthyroidism. Stop ashwagandha and retest if they appear.
Timing
Spacing the doses does not mitigate this interaction — the effect is on thyroid output over weeks, not on absorption. Monitor thyroid labs instead.
Sources
— PMID:28829155 — Sharma AK, Basu I, Singh S. Efficacy and Safety of Ashwagandha Root Extract in Subclinical Hypothyroid Patients. J Altern Complement Med 2018;24(3):243-248.
— PMID:25624699 — Gannon JM, Forrest PE, Chengappa KNR. Subtle changes in thyroid indices during a placebo-controlled study of an extract of Withania somnifera. J Ayurveda Integr Med 2014.
— DOI:10.7759/cureus.23325 — Kamal et al. Ashwagandha as a Unique Cause of Thyrotoxicosis Presenting with Supraventricular Tachycardia. Cureus 2022.
— https://www.mskcc.org/cancer-care/integrative-medicine/herbs/ashwagandha — Memorial Sloan Kettering Integrative Medicine Monograph
How it works
Ashwagandha appears to stimulate thyroid output, raising circulating T3 and T4 and lowering TSH. The effect was clearly demonstrated in subclinical hypothyroid patients at 600 mg/day over 8 weeks (Sharma 2018, RCT): TSH fell (p<0.001), T3 rose (p=0.0031), T4 rose (p=0.0096). The biochemical mechanism is not fully established — authors of the 2022 case report flagged both direct glandular stimulation and possible exogenous T3/T4 contamination of the supplement itself.
Who should be careful
Patients on levothyroxine or liothyronine — highest concern; direct additive effect on circulating thyroid hormone.
Patients on antithyroid medication (methimazole, propylthiouracil) — ashwagandha works against the medication.
Patients with prior thyroid disease, even off medication — the 2022 case was a patient off levothyroxine for 2 years.
Pregnancy and pediatrics — not studied; combined with other ashwagandha safety concerns, avoid.
What we don't know
No human RCT has tested direct co-administration of ashwagandha with levothyroxine. Dose-response below 500 mg/day is uncertain. Whether supplement contamination with exogenous T3/T4 contributes to case-report presentations is unresolved.
Why this severity
WARNING, not DANGER: the effect is RCT-demonstrated but gradual and monitorable, all documented thyrotoxicosis resolved on discontinuation, and no RCT directly tests co-administration. DANGER would require documented non-reversible harm.
Evidence quality (GRADE): moderate
Frequently Asked Questions
Can I take ashwagandha and thyroid medication together?
Ashwagandha can raise thyroid hormone levels and stack additively with levothyroxine, risking thyrotoxicosis.. If you take levothyroxine or any thyroid medication, consult your prescribing clinician before adding ashwagandha. If you proceed, retest TSH and free T4 within 4-8 weeks of starting, and again with any dose or brand change. Watch for palpitations, unintentional weight loss, heat intolerance, or anxiety; these suggest the combination has pushed you toward hyperthyroidism. Stop ashwagandha and retest if they appear.
How should I time ashwagandha and thyroid medication?
Spacing the doses does not mitigate this interaction — the effect is on thyroid output over weeks, not on absorption. Monitor thyroid labs instead.
Is this interaction dangerous?
This interaction is rated “Warning” by Formulate. Ashwagandha (Withania somnifera) stimulates thyroid output in humans at 500-600 mg/day over 8 weeks, raising T3 and T4 and lowering TSH. In a double-blind RCT of 50 subclinical hypothyroid patients (Sharma 2018), ashwagandha 600 mg/day significantly lowered TSH (p<0.001) and raised T3 (p=0.0031) and T4 (p=0.0096) versus placebo. In bipolar patients at 500 mg/day (Gannon 2014), free T4 rose by 7-24% in three treated patients, though the between-group difference was not statistically significant. The effect appears to depend on baseline thyroid state: a separate safety RCT in healthy adults at 300 mg twice daily found no significant thyroid change. No RCT has directly co-administered ashwagandha with levothyroxine. A 2022 case report (Cureus) documented thyrotoxicosis with supraventricular tachycardia in a 73-year-old woman after two years of daily ashwagandha; the authors flagged either glandular stimulation or possible exogenous T3/T4 contamination of the supplement. Memorial Sloan Kettering's monograph notes ashwagandha 'may increase thyroxine levels' and catalogs multiple thyrotoxicosis case reports. For a patient on stable levothyroxine, the plausible effect size (7-24% T4 rise) is large enough to move a well-titrated patient into a suppressed-TSH range. Timing-based workarounds do not help — the effect is endocrine over weeks, not absorption-based. Populations to watch: anyone on thyroid hormone replacement or antithyroid drugs, anyone with a history of thyroid disease even if off medication, and pregnancy/pediatrics (not studied, avoid).
Studies from our registry that mention both ashwagandha and thyroid medication. Each links to the primary source and the other Formulate pages citing it.
The free checker lets you add any combination of supplements and medications at once. The free web app goes further — every stack change surfaces interaction alerts automatically.