Although there are a number of studies that suggest ashwagandha has thyroid-stimulating effects, the jury is still out on whether this effect is caused by supplement contamination with thyroid hormones, as opposed to ashwagandha itself.[1] There are at least three case reports where people developed thyrotoxicosis (excessively high thyroid hormone levels) from supplementing with ashwagandha.[2] There are also at least four case reports that documented a variable, modest increase in thyroid hormone levels.[3] However, there are no clinical trials testing ashwagandha in people with overt hypothyroidism. In one 8-week randomized controlled trial of 50 patients with subclinical hypothyroidism, taking 300 mg of ashwagandha root extract twice daily increased thyroxine levels by 20% while levels of thyroid-stimulating hormone (TSH) decreased by 17% compared to a starch placebo.[4] However, in another 8-week randomized controlled trial, the same dose and type of ashwagandha root extract did not affect thyroid hormones in euthyroid individuals.[5]