Ashwagandha: Cortisol, Thyroid, and Evidence-Based Dosing

Key Takeaways:

• Ashwagandha has moderate evidence from multiple RCTs supporting cortisol reduction of 15 to 30 percent in chronically stressed individuals
• The thyroid-stimulating effect is real and clinically relevant — it can increase T4 and T3 levels, which matters for patients with thyroid conditions
• Standardized root extracts (KSM-66 and Sensoril are the most studied) at 300 to 600mg daily have the most clinical data behind them
• Ashwagandha is one of the few adaptogens where the evidence actually supports the marketing — but the nuance matters
• Quality and contamination are significant concerns, particularly heavy metal content from soil accumulation

At a Glance

Ashwagandha: What the Research Actually Says

If you have spent any time in the supplement space, you have encountered ashwagandha. It is everywhere — in stress formulas, sleep stacks, testosterone boosters, pre-workouts, and nootropic blends. The marketing ranges from reasonable to absurd. What I want to do here is separate what the evidence actually supports from what the supplement industry has extrapolated.

Here is my assessment: ashwagandha is one of the better-studied herbal supplements, and the data does support several of its traditional uses, particularly for stress and anxiety. It is not a miracle compound, but it is also not placebo. The clinical evidence is moderate and consistent enough that I consider it a reasonable option for specific indications. But there are important caveats — particularly around thyroid effects and product quality — that most discussions overlook.

The Plant and Its Active Compounds

Withania somnifera, commonly called ashwagandha or Indian ginseng, is a nightshade family plant that has been used in Ayurvedic medicine for over 3,000 years. The name translates roughly to “smell of the horse” — referring both to the root’s odor and the traditional belief that it confers the strength and vitality of a horse.

The primary active compounds are withanolides, a group of steroidal lactones. Over 40 withanolides have been identified, with withaferin A and withanolide D being the most pharmacologically studied. Different plant parts (root, leaf, whole plant) contain different withanolide profiles, which is why the specific extract and standardization matters clinically.

Root extracts are the most traditional and most studied for stress, anxiety, and hormonal effects. KSM-66 is a full-spectrum root extract standardized to a minimum 5% withanolides. Sensoril is a root and leaf extract standardized to a minimum 10% withanolides (different withanolide profile).

Leaf extracts contain higher concentrations of withaferin A, which has more cytotoxic properties. This is relevant for potential anti-cancer research but also means leaf extracts have a different safety profile than root extracts. Some supplements use leaf-heavy or whole-plant extracts to boost withanolide percentage numbers on the label while providing a fundamentally different product than what was studied in clinical trials.

This is what the supplement industry does not want to talk about: “ashwagandha” on a label tells you almost nothing. The extract type, plant part, standardization, and withanolide profile determine what you are actually taking.

Cortisol and the HPA Axis

The Mechanism

Ashwagandha’s effect on cortisol is its best-studied and most clinically relevant property. The mechanism involves modulation of the hypothalamic-pituitary-adrenal (HPA) axis — the system that controls your stress hormone response.

The HPA axis works as follows: the hypothalamus releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to release adrenocorticotropic hormone (ACTH), which signals the adrenal glands to produce cortisol. This cascade is essential for survival — cortisol mobilizes energy, sharpens focus, and modulates immune function during acute stress. The problem is chronic activation, where cortisol remains elevated for weeks, months, or years, contributing to sleep disruption, metabolic dysfunction, immune suppression, and neurological effects.

Ashwagandha appears to modulate this axis at multiple levels. Withanolides have demonstrated effects on GABA receptors (which have inhibitory effects on HPA activation), on cortisol synthesis enzymes, and on feedback sensitivity of the HPA axis. The net effect is not cortisol suppression — it is improved regulation, bringing chronically elevated cortisol closer to normal ranges.

What the Human Data Shows

The evidence supports this. Here is what I consider the most relevant clinical data.

Chandrasekhar et al. (2012) — A 60-day randomized, double-blind, placebo-controlled trial in 64 adults with chronic stress. The ashwagandha group (300mg KSM-66, twice daily) showed a 27.9% reduction in serum cortisol compared to 7.9% in the placebo group. Perceived Stress Scale scores decreased significantly in the treatment group. This is the most cited cortisol study and remains one of the stronger data points.

Salve et al. (2019) — An 8-week RCT in 58 adults. Both 250mg and 600mg daily doses of ashwagandha root extract significantly reduced serum cortisol (by approximately 15% and 23% respectively) compared to placebo. Sleep quality also improved, measured by the Pittsburgh Sleep Quality Index.

Lopresti et al. (2019) — A 60-day RCT in 60 adults with self-reported high stress. Ashwagandha (240mg Shoden extract daily) reduced morning cortisol by 23% compared to placebo. DHEA-S (a counter-regulatory hormone to cortisol) increased in the treatment group.

Choudhary et al. (2017) — An 8-week RCT in 50 adults. KSM-66 at 300mg twice daily reduced cortisol and improved scores on multiple stress and well-being questionnaires.

The evidence is consistent: multiple RCTs from independent groups show cortisol reduction in the range of 15 to 30 percent in chronically stressed individuals. This is moderate evidence — the studies are relatively small, most come from Indian research groups (reasonable given the plant’s origin, but replication from diverse groups strengthens confidence), and the duration is limited to 8 to 12 weeks in most cases.

What I tell my patients: the cortisol-lowering effect is real and clinically meaningful for people with chronically elevated cortisol. It is not going to fix a cortisol problem caused by unaddressed chronic stressors (a terrible job, a toxic relationship, sleep deprivation) — it is a tool, not a solution.

Thyroid Effects: The Conversation Most People Skip

This is where I spend the most time with patients, because the thyroid effects of ashwagandha are clinically relevant and frequently ignored by supplement marketers.

What the Data Shows

Multiple studies have documented increases in thyroid hormones with ashwagandha supplementation:

Sharma et al. (2018) — An 8-week RCT in 50 subjects with subclinical hypothyroidism. Ashwagandha root extract (600mg daily) significantly increased T3 and T4 levels and normalized TSH compared to placebo. T4 increased by approximately 19.6% and T3 by approximately 41.5%.

Gannon et al. (2014) — A review of ashwagandha’s endocrine effects reported consistent thyroid-stimulating activity across animal and human studies.

Additional case reports have described thyrotoxicosis (excessive thyroid hormone levels) in patients taking ashwagandha supplements.

Clinical Implications

For patients with subclinical hypothyroidism — mildly underactive thyroid with elevated TSH but normal T4 — ashwagandha may provide a modest, clinically relevant improvement. This is where the supplement industry sees a marketing opportunity.

But the same thyroid-stimulating property creates significant concerns for:

• Patients with Graves disease or hyperthyroidism — ashwagandha could worsen an already overactive thyroid
• Patients on thyroid medication (levothyroxine) — ashwagandha may alter thyroid hormone levels, requiring dose adjustment and monitoring
• Patients with thyroid nodules — stimulating thyroid activity in the presence of autonomous nodules warrants caution
• Hashimoto’s thyroiditis patients — this is complex. Some patients with Hashimoto’s are hypothyroid and might benefit, but autoimmune thyroid disease is unpredictable, and stimulating thyroid function during active autoimmune flares could be counterproductive

What I tell my patients: if you are going to take ashwagandha, get your thyroid function tested first (TSH, free T4, free T3 at minimum). If you have any thyroid condition, discuss it with your physician before starting. And if you are on thyroid medication, do not add ashwagandha without medical supervision.

Anxiety and Mental Health

The Evidence

Ashwagandha’s anxiolytic (anti-anxiety) effects have been evaluated in several RCTs:

Pratte et al. (2014) — A systematic review of five RCTs concluded that ashwagandha consistently reduced anxiety scores compared to placebo across studies, though effect sizes varied. The Hamilton Anxiety Rating Scale (HAM-A) showed significant improvement in the treatment groups.

Langade et al. (2019) — A 10-week RCT in 150 healthy adults found that 120mg ashwagandha extract daily significantly reduced General Health Questionnaire-28 scores and DASS-21 anxiety subscale scores compared to placebo.

Cooley et al. (2009) — A 12-week naturalistic study comparing ashwagandha-based naturopathic care to psychotherapy-based conventional care in patients with moderate to severe anxiety. The naturopathic care group (which included 300mg ashwagandha twice daily among other interventions) showed greater improvement in BAI scores.

The evidence supports a genuine anxiolytic effect, likely mediated through GABAergic mechanisms and HPA axis modulation. The effect size is modest compared to pharmaceutical anxiolytics (benzodiazepines, SSRIs) but is meaningful, particularly for mild to moderate anxiety.

In my clinical experience, ashwagandha is most useful for patients whose anxiety is closely linked to chronic stress and cortisol dysregulation — the type of anxiety that worsens with prolonged stress, disrupts sleep, and creates a self-reinforcing cycle. It is less useful for acute anxiety or panic disorder.

Sleep

Multiple studies report improved sleep quality with ashwagandha supplementation. Langade et al. (2019) demonstrated significant improvements in sleep onset latency (how long it takes to fall asleep) and sleep quality in both healthy volunteers and insomnia patients taking 300mg KSM-66 twice daily for 10 weeks.

The sleep effect appears to be partly independent of the cortisol effect — suggesting that ashwagandha’s GABAergic activity may contribute directly to sleep onset and maintenance. For patients with stress-related insomnia, this makes it a particularly logical choice.

Physical Performance and Testosterone

Strength and Recovery

Several RCTs have evaluated ashwagandha for physical performance:

Wankhede et al. (2015) — An 8-week RCT in 57 young men during resistance training. KSM-66 (300mg twice daily) was associated with significantly greater increases in muscle strength (bench press, leg extension) and muscle size compared to placebo. Testosterone increased modestly, and muscle damage markers (creatine kinase) were lower in the ashwagandha group.

Ziegenfuss et al. (2018) — A 12-week RCT in 43 men and women during resistance training. Sensoril (500mg daily) was associated with greater improvements in power output, mean velocity during lifts, and testosterone/cortisol ratios.

The data is promising but limited. The effect sizes are modest, the studies are small, and the athletic performance literature for ashwagandha is nowhere near as robust as the stress and anxiety literature.

Testosterone

The testosterone-boosting claims are among the most aggressively marketed and deserve careful analysis. Several studies show modest testosterone increases — typically in the range of 10 to 20 percent — but these are mostly in populations with suboptimal baseline testosterone (stressed, sedentary, subfertile men). Whether ashwagandha meaningfully increases testosterone in healthy men with normal levels is less clear.

What I tell my patients: if your testosterone is low partly because your cortisol is chronically high, ashwagandha may help by improving the cortisol-to-testosterone balance. It is not going to compete with TRT for someone with genuinely low testosterone from primary or secondary hypogonadism.

Practical Application: Dosing and Extract Selection

Which Extract

KSM-66 (standardized to minimum 5% withanolides from root only): The most studied extract for stress, cortisol, anxiety, sleep, and physical performance. This is my default recommendation.

Sensoril (standardized to minimum 10% withanolides from root and leaf): Well-studied for cortisol reduction and stress. Some evidence for physical performance. The higher withanolide percentage does not necessarily mean it is “stronger” — the withanolide profile is different from KSM-66.

Shoden (standardized to minimum 35% withanolide glycosides): A newer, high-potency extract used at lower doses (120-240mg). Limited but promising data.

Generic “ashwagandha root extract”: Quality is unpredictable. Without standardization to specific withanolides and third-party testing, you cannot know what you are getting.

Dosing

Based on the clinical literature:

• KSM-66: 300mg twice daily (600mg total) or 600mg once daily. This is the most-studied dosing.
• Sensoril: 125 to 250mg twice daily.
• Shoden: 120 to 240mg once daily.
• Timing: For cortisol and anxiety, split dosing (morning and evening) may be preferable. For sleep, evening dosing with dinner. It is typically taken with food, which may improve absorption and reduce GI effects.

Cycling

I generally recommend cycling ashwagandha rather than continuous use. A common protocol is 8 to 12 weeks on, 2 to 4 weeks off. This is not based on strong evidence for cycling specifically but on general principle: chronic supplementation without breaks makes it difficult to assess whether the supplement is still providing benefit, and some data suggests adaptation over time.

Safety and Considerations

Generally Well-Tolerated

In clinical trials, ashwagandha has a good safety profile at standard doses for durations up to 12 weeks. Common side effects include:

• Mild gastrointestinal discomfort (usually resolves within a few days)
• Drowsiness (more common with evening dosing — this can be a feature, not a bug, for sleep)
• Mild headache (uncommon)

Contraindications and Cautions

Thyroid disease. Already discussed in detail above. Monitor thyroid function.

Autoimmune conditions. Ashwagandha has immunomodulatory properties that may stimulate immune function. For patients with autoimmune conditions (lupus, rheumatoid arthritis, multiple sclerosis, and others), immune stimulation could theoretically worsen disease activity. The clinical data on this is limited, but caution is warranted.

Pregnancy and breastfeeding. Insufficient safety data. Traditional Ayurvedic texts contraindicate ashwagandha in pregnancy (classified as an abortifacient in some traditions). Avoid.

Nightshade sensitivity. Ashwagandha is a Solanaceae family member. Patients who react to nightshades (tomatoes, peppers, eggplant) may also react to ashwagandha.

Medication interactions. May potentiate sedatives (benzodiazepines, barbiturates), thyroid medications, immunosuppressants, and potentially blood sugar-lowering medications. Discuss with your prescriber.

Liver toxicity. Rare case reports of hepatotoxicity associated with ashwagandha supplements have been published (Bjornsson et al., 2020). Whether this is caused by the ashwagandha itself, contaminants, or other factors is not clear. Monitoring liver function during prolonged use is reasonable.

The Quality Problem

Here is where my anger at the supplement industry becomes relevant. Ashwagandha is one of the supplements where quality variation is most problematic.

Heavy metals. Ashwagandha is a bioaccumulator — it concentrates heavy metals (lead, arsenic, mercury, cadmium) from the soil where it is grown. Products sourced from contaminated agricultural regions can contain heavy metal levels that exceed safe limits for daily consumption. This is not hypothetical — it has been documented repeatedly in independent testing.

Withanolide content. Products claiming “500mg ashwagandha” may contain vastly different amounts of active withanolides depending on the extract type, standardization, and quality control.

Adulteration. Less expensive ashwagandha products may be adulterated with other plant material or synthetic withanolides.

What I tell my patients: buy a branded extract (KSM-66 or Sensoril) from a manufacturer that provides third-party testing certificates. Check for USP, NSF, or ConsumerLab verification. The price difference between a quality ashwagandha product and a low-quality one is typically a few dollars per month — not worth the risk.

The Bottom Line

Ashwagandha has earned its place as one of the better-evidenced adaptogenic herbs. The cortisol-lowering, anxiolytic, and sleep-promoting effects are supported by moderate clinical evidence from multiple independent RCTs. The thyroid-stimulating effect is real and clinically significant — helpful for some, risky for others. Use a standardized, tested extract at evidence-based doses, cycle your use, monitor your thyroid function, and buy quality products. It is a useful tool for stress management, not a substitute for addressing the underlying causes of chronic stress.

References

1. Chandrasekhar K, et al. 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 J Psychol Med. 2012;34(3):255-262. doi:10.4103/0253-7176.106022
2. Salve J, et al. Adaptogenic and anxiolytic effects of ashwagandha root extract in healthy adults: A double-blind, randomized, placebo-controlled clinical study. Cureus. 2019;11(12):e6466. doi:10.7759/cureus.6466
3. Lopresti AL, et al. An investigation into the stress-relieving and pharmacological actions of an ashwagandha (Withania somnifera) extract: A randomized, double-blind, placebo-controlled study. Medicine. 2019;98(37):e17186.
4. Sharma AK, et al. Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: A double-blind, randomized placebo-controlled trial. J Altern Complement Med. 2018;24(3):243-248. doi:10.1089/acm.2017.0183
5. Langade D, et al. Clinical evaluation of the pharmacological impact of ashwagandha root extract on sleep in healthy volunteers and insomnia patients. J Ethnopharmacol. 2020;264:113276.
6. Wankhede S, et al. Examining the effect of Withania somnifera supplementation on muscle strength and recovery: A randomized controlled trial. J Int Soc Sports Nutr. 2015;12:43. doi:10.1186/s12970-015-0104-9
7. Pratte MA, et al. An alternative treatment for anxiety: A systematic review of human trial results reported for the Ayurvedic herb ashwagandha. J Altern Complement Med. 2014;20(12):901-908.
8. Bjornsson HK, et al. Ashwagandha-induced liver injury: A case series from Iceland and the US Drug-Induced Liver Injury Network. Liver Int. 2020;40(4):825-829.