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