Ashwagandha and Thyroid Medications: Risk of Over-Replacement Explained

Many people turn to ashwagandha to manage stress, improve sleep, or boost energy. But if you're taking thyroid medication like levothyroxine (Synthroid), this popular herb could be quietly pushing your thyroid hormone levels too high-leading to a dangerous condition called over-replacement.

What is over-replacement, and why does it matter?

Over-replacement happens when your thyroid hormone levels rise beyond what your body needs, even though you're taking the right dose of medication. It’s not a mistake in prescribing-it’s often caused by something else you’re taking. Ashwagandha is one of those hidden triggers.

Your thyroid gland makes two main hormones: T4 (thyroxine) and T3 (triiodothyronine). When you have hypothyroidism, your doctor prescribes a synthetic version of T4, usually levothyroxine, to bring your levels back to normal. The goal is to keep your TSH (thyroid-stimulating hormone) between 0.4 and 4.0 mIU/L. Too low, and you’re in hyperthyroid territory-your heart races, you lose weight without trying, you can’t sleep, and your bones weaken over time.

Ashwagandha doesn’t just calm you down. It also stimulates your thyroid. A 2018 study of 50 people with subclinical hypothyroidism found that taking 600 mg of ashwagandha daily for eight weeks raised T4 by nearly 20%, T3 by over 40%, and lowered TSH significantly. That’s not a mild effect. That’s the same kind of change your doctor tries to achieve with medication. When you add ashwagandha to levothyroxine, you’re essentially doubling down on thyroid stimulation.

How ashwagandha affects thyroid hormones

Ashwagandha contains compounds called withanolides, especially withaferin A and withanolide D. These don’t just reduce cortisol-they interact directly with your hypothalamic-pituitary-thyroid (HPT) axis. In simple terms, they signal your brain to tell your thyroid to produce more hormones. Some lab studies show it can even boost thyroid peroxidase (TPO) activity by up to 38%, the enzyme your thyroid needs to make T4 and T3.

The problem? Your body doesn’t know the difference between a hormone made by your thyroid and one made by a pill. So if ashwagandha pushes your natural production up while you’re already taking levothyroxine, your total hormone levels skyrocket. The American Association of Clinical Endocrinologists documented 12 cases of thyrotoxicosis-dangerously high thyroid hormone levels-linked to ashwagandha use in people on thyroid meds. In these cases, T4 levels hit over 25 mcg/dL. The normal range is 4.5 to 12.0. That’s more than double the upper limit.

Real patient stories: What happens when it goes wrong

On thyroid forums, the stories are consistent. One user on Thyroid Help Forum, who took 500 mg of ashwagandha daily with 100 mcg of levothyroxine, saw their TSH crash from 1.8 to 0.08 mIU/L in six weeks. They developed palpitations, anxiety, and insomnia. They ended up in the ER. Another survey of over 1,200 thyroid patients found that nearly 19% who took ashwagandha experienced symptoms of hyperthyroidism. Nearly 30 of them needed hospital care for heart rhythm problems.

It’s not just about feeling jittery. Over-replacement increases your risk of atrial fibrillation, bone loss, and even heart failure over time. And because ashwagandha stays in your system for weeks-its half-life is about 12 days-your thyroid levels might stay elevated even after you stop taking it. That means a routine blood test taken just a few days after quitting ashwagandha could still show abnormal results, leading your doctor to think your medication dose is too high when it’s actually the supplement causing the problem.

The supplement industry isn’t helping

Ashwagandha supplements are sold in pharmacies, grocery stores, and online with no regulation on potency. The FDA doesn’t test them before they hit shelves. ConsumerLab.com tested 15 popular brands in 2021 and found withanolide content varied from 1.2% to 7.8%-more than six times different between products. One bottle might have enough to boost your thyroid. Another might do nothing. You can’t tell by the label.

And the marketing? Many brands claim ashwagandha “supports thyroid health” without proof. The FDA has issued 12 warning letters to manufacturers for making false thyroid claims since 2020. But enforcement is weak. You can still buy a bottle that says “for thyroid support” right next to your levothyroxine at the checkout.

What doctors say: Clear warnings, but not enough

Endocrinologists are united on this. The Endocrine Society, Mayo Clinic, and UCLA’s endocrine clinic all advise against using ashwagandha if you’re on thyroid medication. Dr. Angela Leung of UCLA says it can tip the “delicate balance” of hormone replacement, causing iatrogenic hyperthyroidism-that means the treatment itself is causing the problem.

Even integrative medicine experts who support herbal remedies draw the line here. Dr. Mary Hardy at Cedars-Sinai says the therapeutic window for thyroid meds is razor-thin. Adding an unregulated herb is like driving with your foot on both the gas and the brake. You might think you’re in control, but you’re not.

The only exception? People not on any thyroid meds who have mild, untreated hypothyroidism. Some report feeling better on ashwagandha alone. But that’s not a substitute for medical care. And if you start taking thyroid medication later, you’ll need to stop ashwagandha immediately.

What you should do right now

If you’re taking levothyroxine, liothyronine, or any thyroid medication:

• Stop taking ashwagandha. Don’t wait for symptoms. The risk isn’t worth it.
• Talk to your doctor. Let them know you’ve been using it-even if you stopped. They may need to retest your thyroid levels.
• Wait 30 days before a thyroid blood test. Ashwagandha can skew results for weeks after you quit. Your doctor needs an accurate baseline.
• Check your supplement labels. If it says “supports thyroid function,” put it back. That’s not a benefit-it’s a warning sign.

Alternatives for stress and sleep

You don’t need ashwagandha to manage stress. There are safer options:

• Magnesium glycinate-helps with relaxation and sleep, no thyroid interaction.
• L-theanine-an amino acid in green tea that reduces anxiety without affecting hormones.
• Cognitive behavioral therapy (CBT)-proven to lower cortisol and improve sleep long-term.
• Regular exercise and sleep hygiene-the most effective, drug-free ways to manage stress.

The bottom line

Ashwagandha isn’t evil. It’s a powerful herb with real benefits-for some people, in some situations. But if you’re on thyroid medication, it’s not a supplement you can safely experiment with. The science is clear: it raises your thyroid hormone levels. Combine that with a prescription designed to do the same thing, and you’re playing with fire.

Your thyroid is a finely tuned system. Don’t let an unregulated herb throw it out of balance. The risk of heart problems, bone loss, and hospital visits isn’t worth feeling a little less stressed. Talk to your doctor. Find safer alternatives. Your thyroid will thank you.