Before your blood test
Does ashwagandha affect thyroid blood tests?
Short answer
It can. Ashwagandha has been shown in small trials to raise thyroid hormone levels (T4 and T3) and lower TSH in people with an underactive thyroid, and case reports describe over-active thyroid results in people taking it — so a thyroid panel drawn while you take it may not reflect your untreated baseline. Many 'thyroid support' blends add iodine, selenium, or biotin, and biotin separately interferes with thyroid immunoassays. If the test is meant to establish or re-check a thyroid diagnosis, tell the clinician what you take and when you last took it; do not stop prescribed thyroid medication.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| TSH | can lower | While taking; the studies that measured it ran for about eight weeks | Note the supplement on a thyroid panel; a clinician may repeat TSH after a planned pause [source] |
| Free T4, Free T3, total T4/T3 | can raise | While taking | Interpret with the supplement in mind; stop before a repeat draw only with the clinician's agreement [source] |
| Thyroid results on biotin-containing 'thyroid support' blends | interferes | 2–3 days after the last high biotin dose | Check the label for biotin; stop ≥72 hours before the draw (see Biotin) [source] |
| Cortisol | can lower | While taking; reported in stress-focused trials | Mention it when a morning cortisol is being interpreted [source] |
| ALT, AST (rare liver injury reports) | can raise | Case reports; usually resolves after stopping | An unexplained rise in liver enzymes on ashwagandha warrants telling the clinician [source] |
Why would a herb change a thyroid panel?
Ashwagandha (Withania somnifera) appears to nudge thyroid hormone output upward. In small randomized trials in people with subclinical hypothyroidism, eight weeks of a root extract raised T4 and T3 and lowered TSH compared with placebo. Published case reports also describe thyrotoxicosis — high thyroid hormone with symptoms — in people taking it, which resolved after stopping. The effect is not large or universal, but it is in the direction that can make an underactive thyroid look better treated than it is, or a normal thyroid look mildly over-active.
That matters for two decisions: whether you have hypothyroidism at all, and whether a levothyroxine dose is right. Both are judged from TSH, and a supplement that moves TSH blurs the picture. It does not mean the result is 'wrong' — it reflects your body on the supplement — but the clinician needs to know it is there.
What about the rest of a 'thyroid support' formula?
Many blends combine ashwagandha with iodine, selenium, zinc, L-tyrosine and biotin. Iodine in excess can push thyroid function either way in susceptible people. Biotin is a different problem: it does not change your thyroid at all, but it interferes with the chemistry of many thyroid immunoassays and can produce a falsely low TSH with falsely high free T4 — a pattern that mimics hyperthyroidism. Stop biotin-containing products at least 72 hours before a thyroid draw; for ashwagandha itself, the honest answer is to tell the clinician and let them decide whether a repeat off the supplement is needed.
Common questions
Should I stop ashwagandha before a thyroid blood test?
Tell the ordering clinician you take it and when you last took it. If the test is to diagnose a thyroid problem or set a medication dose, they may ask for a repeat after a planned pause. Never stop prescribed levothyroxine to 'clean up' a result.
Can ashwagandha cause a low TSH?
Yes — small trials in people with a mildly underactive thyroid showed lower TSH and higher T4/T3 after about eight weeks. In someone with a normal thyroid the shift is usually small, but case reports of over-active results exist.
Does ashwagandha affect other blood tests?
Stress-focused trials reported lower cortisol, and rare case reports describe liver-enzyme rises that resolved after stopping. Neither is common; both are worth mentioning if a result is unexpected.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.