Evidence-graded guideReviewed October 31, 2026

Herbs and Supplements for Thyroid Health, Hypothyroidism and Hashimoto's: What the Evidence Supports

The supplements with the best evidence for thyroid health are selenium, which lowers thyroid peroxidase (TPO) antibodies modestly in Hashimoto's across more than a dozen RCTs, vitamin D, which is deficient in most people with autoimmune thyroid disease and improves antibody levels when corrected, and iron, because iron deficiency directly impairs thyroid hormone production and is the most common reason levothyroxine seems to stop working. Myo-inositol combined with selenium and, for subclinical hypothyroidism specifically, ashwagandha come next. None of these replace thyroid hormone.

That point needs saying clearly because of what is sold online. If you have overt hypothyroidism or Hashimoto's with a raised TSH, levothyroxine is not optional, and no herb restores a gland that autoimmunity has damaged. What supplements can do is correct the nutrient deficiencies that make conversion of T4 to T3 sluggish, modestly reduce the autoimmune attack, and in subclinical cases nudge TSH down. Two warnings run through this whole page: iodine supplements can worsen Hashimoto's and tip people into hypo- or hyperthyroidism, and biotin, found in most hair and nail supplements, falsifies thyroid blood tests for up to 3 days, producing results that look like hyperthyroidism.

This guide is for adults with diagnosed hypothyroidism, Hashimoto's or subclinical hypothyroidism who want to know what is worth adding alongside medication. See a clinician rather than a supplement if you have symptoms of thyroid disease that have never been tested (fatigue, weight change, cold intolerance, hair loss, constipation, heavy periods), a lump or swelling in the neck, palpitations, tremor or unexplained weight loss (possible hyperthyroidism), or if you are pregnant or planning to be, because thyroid needs change fast and iodine and selenium advice differs. Every supplement here should be taken at least 4 hours apart from levothyroxine.

Quick answer

Selenium — 200 mcg/day lowers TPO antibodies by roughly 20–40% in Hashimoto's over 3–12 months; does not change TSH much
Vitamin D — deficient in most people with Hashimoto's; correcting it lowers antibodies and may reduce TSH when low
Iron — ferritin below 50–70 ng/mL impairs thyroid hormone synthesis; correction improves TSH and symptoms
Myo-inositol + selenium — lowered TSH and antibodies in subclinical Hashimoto's over 6 months
Ashwagandha — 600 mg/day lowered TSH and raised T4 in an 8-week subclinical hypothyroidism RCT
Zinc — needed for T3 receptor function; helps when levels are low, particularly with hair loss

The 10 best herbs and supplements for thyroid

Ranked by the strength of human evidence. Strong = multiple trials or a meta-analysis. Moderate = a few small trials. Preliminary = early or animal data.

Selenium
Strong evidence
The thyroid has the highest selenium concentration of any organ, and selenoproteins both make T3 and protect the gland from hydrogen peroxide generated during hormone synthesis. A 2016 meta-analysis of 16 RCTs and a 2023 meta-analysis of 35 RCTs found 200 mcg/day lowered TPO antibodies by around 20–40 percent in Hashimoto's at 3–12 months, improved wellbeing scores, and in one 2002 RCT shrank antibody levels by 40 percent in 3 months. However, Cochrane rated the evidence low quality, and selenium does not reliably lower TSH or reduce levothyroxine doses. In Graves' disease it may speed remission in selenium-deficient people.
Key evidence
2023 meta-analysis of 35 RCTs on TPO antibodies and TSH in Hashimoto's thyroiditis (Huwiler et al.)
Studied dose
100–200 mcg/day of selenomethionine or selenium yeast, 3–12 months; two Brazil nuts a day provide roughly the same
Best for
Hashimoto's with high TPO antibodies, especially in regions with low-selenium soil (much of Europe)
Cautions
Do not exceed 400 mcg/day from all sources: chronic excess causes hair loss, brittle nails, garlic breath and, in one large trial, higher type 2 diabetes risk. Keep 4 hours from levothyroxine. Stop before surgery if advised.
Full Selenium profile
Vitamin D3
Moderate evidenceCholecalciferol (D3)
Vitamin D deficiency is found in 60–90 percent of people with Hashimoto's in observational studies, and lower levels correlate with higher antibodies. A 2021 meta-analysis of 7 RCTs found supplementation lowered TPO antibodies, and a 2019 meta-analysis of 11 trials found reductions in both TPO and thyroglobulin antibodies, with a small drop in TSH in some trials. A 2016 RCT of 60,000 IU weekly for 8 weeks in 100 patients lowered TSH significantly compared with placebo. Benefits appear only when correcting a real deficiency.
Key evidence
2021 meta-analysis of 7 RCTs on thyroid autoantibodies in Hashimoto's (Zhang et al.)
Studied dose
1,000–4,000 IU/day D3 with a fatty meal to bring 25-OH-D above 30 ng/mL (75 nmol/L); recheck after 3 months
Best for
Hashimoto's or hypothyroidism with a low level, which is most people with it
Cautions
High doses without testing can raise calcium. Interacts with thiazide diuretics and digoxin. Supervised use in sarcoidosis, granulomatous disease or hyperparathyroidism. Safe in pregnancy at standard doses.
Full Vitamin D3 profile
Iron (when ferritin is low)
Moderate evidence
Thyroid peroxidase is an iron-dependent enzyme, so iron deficiency reduces thyroid hormone synthesis and T4-to-T3 conversion. Iron-deficient women have higher TSH and lower T4, and a 2024 systematic review found correcting deficiency improved TSH and thyroid hormone levels. Low ferritin is also a common reason for persistent fatigue and hair loss in treated hypothyroidism, and it is frequent because hypothyroidism itself causes heavy periods and reduces stomach acid. Aim for ferritin above 50–70 ng/mL.
Key evidence
2024 systematic review of iron supplementation and thyroid function in iron deficiency; Zimmermann's RCTs of iron with iodine in goitrous children
Studied dose
Only after a ferritin test: 40–80 mg elemental iron on alternate days with vitamin C, recheck in 3 months
Best for
Menstruating women, people with Hashimoto's plus coeliac or low stomach acid, anyone with persistent fatigue despite a normal TSH
Cautions
Iron blocks levothyroxine absorption; separate by at least 4 hours or your TSH will rise. Never take iron without a blood test (overload, haemochromatosis). Constipation and black stools are common. Keep away from children.
Full Iron (when ferritin is low) profile
Myo-inositol (with selenium)
Moderate evidence
Myo-inositol is part of the TSH signalling pathway inside thyroid cells. A 2017 RCT of 168 people with subclinical Hashimoto's found 600 mg myo-inositol plus 83 mcg selenium daily for 6 months lowered TSH by about 30 percent, lowered TPO and thyroglobulin antibodies, and improved quality of life, while selenium alone lowered antibodies without the TSH change. A 2013 pilot and a 2018 follow-up reported similar results. All the trials come from one Italian group using a single product, so independent replication is needed.
Key evidence
2017 RCT of myo-inositol plus selenium vs selenium alone in 168 patients with subclinical hypothyroidism (Nordio & Basciani)
Studied dose
600 mg/day myo-inositol with 83 mcg selenium, for 6 months
Best for
Subclinical hypothyroidism with positive antibodies, where TSH is 4–10 and medication has not yet been started
Cautions
Very safe; mild nausea at high doses. Can lower blood sugar with diabetes drugs. Keep 4 hours from levothyroxine.
Full Myo-inositol (with selenium) profile
Ashwagandha
Moderate evidenceWithania somnifera
Ashwagandha appears to stimulate thyroid hormone output. A 2018 RCT of 50 adults with subclinical hypothyroidism found 600 mg/day of root extract for 8 weeks lowered TSH by about 17 percent and raised T4 by 20 percent and T3 by 40 percent compared with placebo, and a bipolar-disorder trial found a similar T4 rise. These are small trials, and the same effect means ashwagandha can push a treated patient into mild hyperthyroidism; case reports of thyrotoxicosis exist. It is not for Hashimoto's with normal TSH on medication, and not for hyperthyroidism at all.
Key evidence
2018 RCT of ashwagandha 600 mg/day for 8 weeks in 50 adults with subclinical hypothyroidism (Sharma et al.)
Studied dose
300 mg twice daily of a root extract standardised to withanolides, 8 weeks, with TSH rechecked at the end
Best for
Subclinical hypothyroidism not yet on medication, particularly with stress and poor sleep
Cautions
Can cause overtreatment in people on levothyroxine; recheck TSH after 8 weeks. Avoid in hyperthyroidism and Graves'. Rare liver injury reports; stop if jaundiced. Avoid in pregnancy, autoimmune flares and before surgery; adds to sedatives.
Full Ashwagandha profile
Zinc
Preliminary evidence
Zinc is needed for thyroid hormone synthesis, T3 receptor binding and the deiodinase enzymes that convert T4 to T3, and hypothyroidism in turn reduces zinc absorption. A 2015 RCT in 68 overweight women with hypothyroidism found 30 mg/day zinc for 12 weeks raised free T3, and zinc plus selenium raised free T4 and lowered TSH. Hair loss in hypothyroidism improves with zinc in small studies when levels are low. There are no large trials, and no benefit is expected if you are zinc replete.
Key evidence
2015 RCT of zinc and selenium supplementation on thyroid function in 68 hypothyroid women (Mahmoodianfard et al.)
Studied dose
15–30 mg/day elemental zinc with food, 12 weeks; check copper if continuing longer
Best for
Hypothyroidism with hair loss, low dietary zinc or a vegan diet
Cautions
Over 40 mg/day long term causes copper deficiency. Separate from levothyroxine, quinolone and tetracycline antibiotics by 4 hours. Nausea on an empty stomach.
Full Zinc profile
Magnesium glycinate
Preliminary evidence
Magnesium is a cofactor for converting T4 to T3 and for iodine uptake into the gland. A 2018 study of 1,257 people found that low serum magnesium was associated with 3 to 5 times higher odds of thyroglobulin antibodies and Hashimoto's, and small trials show magnesium improves the muscle cramps, constipation and poor sleep that come with hypothyroidism. There is no RCT showing magnesium changes TSH, so it is supportive rather than disease-modifying.
Key evidence
2018 cross-sectional study of 1,257 patients linking low magnesium to thyroid antibodies and hypothyroidism (Wang et al.)
Studied dose
200–400 mg/day elemental magnesium as glycinate in the evening
Best for
Hypothyroidism with cramps, constipation, anxiety or poor sleep
Cautions
Loose stools above 400 mg; reduce in kidney disease. Separate from levothyroxine by 4 hours (magnesium salts reduce absorption) and from antibiotics and bisphosphonates by 2–4 hours.
Full Magnesium glycinate profile
Vitamin B12
Preliminary evidenceCobalamin
Around 30–40 percent of people with Hashimoto's have B12 deficiency, largely because autoimmune gastritis and pernicious anaemia cluster with it. Deficiency causes fatigue, numbness, low mood and memory problems that are easily blamed on the thyroid. A 2008 study found B12 replacement in deficient hypothyroid patients improved symptoms markedly, though it does not alter thyroid hormone levels. It is a check-and-correct nutrient, not a thyroid treatment.
Key evidence
2008 study of 116 hypothyroid patients finding 40% B12 deficient, with symptomatic improvement on replacement (Jabbar et al.)
Studied dose
1,000 mcg/day oral methylcobalamin or cyanocobalamin if low (below about 300 pg/mL); injections if pernicious anaemia is confirmed
Best for
Hashimoto's with fatigue, tingling or low mood despite a normal TSH; vegans; people on metformin or acid suppressants
Cautions
Very safe. If you take biotin-containing B-complex products, stop them 3 days before thyroid blood tests, because biotin falsifies TSH and T4 results.
Full Vitamin B12 profile
Omega-3 (EPA and DHA)
Preliminary evidence
Omega-3 is anti-inflammatory and hypothyroidism raises LDL and triglycerides, so it addresses a downstream problem rather than the gland. Animal studies show EPA and DHA support thyroid hormone receptor signalling, and a 2024 Mendelian randomisation study linked higher omega-3 to lower hypothyroidism risk, but there is no RCT showing omega-3 lowers TSH or antibodies in humans. It earns a place for cardiovascular risk and dry skin and eyes, not for thyroid function itself.
Key evidence
No human RCT on thyroid outcomes; evidence is mechanistic plus 2024 genetic association data
Studied dose
1–2 g/day combined EPA + DHA with food
Best for
Hypothyroidism with raised triglycerides, dry eyes or joint aches
Cautions
Mild antiplatelet effect; tell your doctor if you take warfarin or DOACs and stop a week before surgery. Fishy burps and loose stools at higher doses.
Full Omega-3 (EPA and DHA) profile
Probiotics (Lactobacillus and Bifidobacterium strains)
Preliminary evidence
Gut bacteria influence iodine and selenium absorption and levothyroxine uptake, and Hashimoto's is linked to intestinal permeability and small intestinal bacterial overgrowth. A 2020 RCT of 60 hypothyroid patients found a multi-strain probiotic for 8 weeks reduced fluctuations in TSH and the need for levothyroxine dose adjustments, though it did not change TSH itself. A 2022 meta-analysis found probiotics improved thyroid hormone levels slightly without affecting antibodies. Early days.
Key evidence
2020 RCT of probiotic supplementation in 60 patients on levothyroxine (Spaggiari et al.)
Studied dose
10–20 billion CFU/day multi-strain product, 8 weeks
Best for
Hypothyroidism with bloating, IBS, or a TSH that bounces around despite good adherence
Cautions
Avoid live probiotics if severely immunocompromised or with a central line. Gas and bloating in the first week are common. Keep away from levothyroxine dosing time.
Full Probiotics (Lactobacillus and Bifidobacterium strains) profile

How to choose

First, get the basics measured: TSH, free T4, TPO antibodies, ferritin, vitamin D, B12 and, if hair loss is an issue, zinc. Most of the value in thyroid supplementation comes from fixing a deficiency, and you cannot know which one without bloods.
If you have Hashimoto's with high TPO antibodies, selenium 200 mcg/day for 6–12 months is the best-supported add-on; expect lower antibodies, not a lower levothyroxine dose.
If your TSH is 4–10, antibodies are positive and you are not yet on medication, the myo-inositol plus selenium combination or ashwagandha 600 mg/day both have RCTs showing TSH falls, and either is reasonable for an 8–24 week trial with a recheck.
If you are already on levothyroxine and still tired, look at ferritin, B12 and vitamin D before blaming the dose; low iron is the most common fixable cause, and it also blocks levothyroxine absorption if taken at the same time.
Do not add iodine unless a clinician has confirmed you are deficient (urinary iodine) or you are pregnant and following standard prenatal advice. In Hashimoto's, extra iodine increases antibody levels and can raise TSH.
Take levothyroxine on an empty stomach with water and keep every supplement here, plus coffee, calcium and antacids, at least 4 hours away from it. Taking them together is the single most common reason thyroid control deteriorates.

What to skip

Iodine supplements and kelp (including bladderwrack)

Iodine deficiency does cause hypothyroidism, but in people with Hashimoto's extra iodine fuels the autoimmune process: a 2014 trial found iodine restriction improved thyroid function in 78 percent of Hashimoto's patients, and high-iodine regions have more autoimmune thyroiditis. Kelp and bladderwrack contain wildly variable iodine (sometimes several thousand mcg per capsule) plus arsenic and heavy metals, and have caused both hypo- and hyperthyroidism in case reports.

Desiccated thyroid and "thyroid support" glandulars bought online

Over-the-counter bovine thyroid glandulars have been found to contain real T3 and T4 in unpredictable amounts, causing thyrotoxicosis. Prescription desiccated thyroid exists, but dosing it is a prescriber's job, not a supplement decision.

Biotin-containing hair, skin and nail supplements before blood tests

Biotin at 5–10 mg interferes with the immunoassays used for TSH, T4 and T3, producing a false picture of hyperthyroidism or Graves' disease. Stop biotin for at least 3 days before any thyroid test, and tell the lab if you take it.

Black seed oil as a thyroid treatment

One 8-week RCT of 2 g/day Nigella sativa in 40 Hashimoto's patients found lower TSH and antibodies, which is interesting, but it is a single small trial from one group and has not been replicated. It is in our library and worth watching, but it does not yet earn a ranking above the nutrients with multiple trials.

Safety and when to see a clinician

Levothyroxine is not optional. If you have overt hypothyroidism, do not reduce or stop it in favour of supplements; untreated hypothyroidism raises cholesterol, causes heart failure and, in pregnancy, harms the baby's brain development.
Separate everything from levothyroxine by at least 4 hours: iron, calcium, magnesium, zinc, selenium, probiotics, fibre, soy and coffee all reduce its absorption. A morning tablet with water and supplements at lunch or bedtime works.
Biotin falsifies thyroid labs. Doses above 1 mg (common in hair and nail products, often 5,000–10,000 mcg) produce falsely low TSH and high T4, mimicking hyperthyroidism. Stop it 3 days before testing.
Iodine can worsen Hashimoto's and trigger both hypothyroidism and hyperthyroidism. Do not take iodine, kelp or bladderwrack unless a clinician has confirmed deficiency; the upper limit is 1,100 mcg/day and many kelp products exceed it.
Ashwagandha raises thyroid hormone output and has caused thyrotoxicosis in people also on levothyroxine; never use it in hyperthyroidism, and recheck TSH after 8 weeks if you add it. Selenium above 400 mcg/day is toxic.
See a clinician urgently for a neck lump, difficulty swallowing, palpitations, tremor, eye bulging or unexplained weight loss. See one soon if you are pregnant or planning pregnancy, because levothyroxine needs rise 25–50 percent in early pregnancy and iodine advice is different.

What matters as much as any herb

Consistent levothyroxine timing: same time daily, empty stomach, 30–60 minutes before food or at bedtime 3 hours after eating. Switching between these habits changes absorption enough to move TSH.
Dietary selenium and iodine from food: two Brazil nuts, eggs, fish and dairy cover selenium and iodine needs for most people without the overshoot risk of supplements. Very low-iodine diets (strict vegan without iodised salt or seaweed) do cause deficiency.
Coeliac screening: coeliac disease is 4–5 times more common in Hashimoto's and silently blocks absorption of levothyroxine, iron and B12. A one-off blood test is worth asking for.
Gentle, regular exercise improves the fatigue, weight and mood of hypothyroidism once TSH is controlled; it does not change hormone levels but it does change how you feel.
Smoking and stress: smoking raises Graves' risk and worsens thyroid eye disease; chronic stress raises cortisol, which blunts T4-to-T3 conversion. Both are modifiable.

Frequently asked questions

What is the best supplement for an underactive thyroid?

Selenium has the most trial support, lowering TPO antibodies in Hashimoto's by 20–40 percent at 200 mcg/day. For most people, though, the biggest gains come from correcting vitamin D, iron or B12 deficiency, which are all common in hypothyroidism and cause the same fatigue and hair loss. None replace levothyroxine.

Can herbs replace levothyroxine?

No. No herb regenerates a thyroid gland damaged by Hashimoto's or surgery, and stopping levothyroxine causes rising cholesterol, heart problems and, in pregnancy, harm to the baby. Ashwagandha can nudge hormone output in subclinical cases but is not a replacement in overt hypothyroidism.

Should I take iodine for hypothyroidism?

Only if you are shown to be deficient. In the UK, US and most of Europe, hypothyroidism is usually autoimmune, not iodine-related, and extra iodine raises antibody levels and can worsen Hashimoto's. Kelp and bladderwrack are especially risky because their iodine content varies wildly.

Does selenium lower thyroid antibodies?

Yes, modestly. Meta-analyses of RCTs show 200 mcg/day lowers TPO antibodies by roughly 20–40 percent over 3–12 months in Hashimoto's. It does not reliably lower TSH or let you reduce levothyroxine, and the evidence has been rated low to moderate quality. Keep total intake under 400 mcg/day.

Is ashwagandha good for thyroid?

In one 8-week RCT of subclinical hypothyroidism, 600 mg/day lowered TSH by about 17 percent and raised T4 and T3. That same action makes it unsafe in hyperthyroidism and risky if you are already on levothyroxine, where it has caused thyrotoxicosis. Use it only with a plan to recheck TSH.

Why does my thyroid blood test look wrong?

The commonest reason is biotin: hair, skin and nail supplements with 5–10 mg biotin produce falsely low TSH and falsely high T4, mimicking hyperthyroidism. Stop biotin for 3 days before testing. Other causes are taking levothyroxine within 4 hours of iron, calcium or coffee, recent illness, or pregnancy.

How far apart should I take supplements and thyroid medication?

At least 4 hours. Iron, calcium, magnesium, zinc, selenium, fibre, soy and even coffee reduce levothyroxine absorption. A practical routine is levothyroxine on waking with water, breakfast 30–60 minutes later, and supplements at lunch or bedtime.

References

van Zuuren EJ et al.. Selenium supplementation for Hashimoto's thyroiditis: summary of a Cochrane systematic review. European Thyroid Journal (2014)View source
Huwiler VV et al.. Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid (2024)
Sharma AK, Basu I, Singh S. Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: a double-blind, randomized placebo-controlled trial. Journal of Alternative and Complementary Medicine (2018)View source
Nordio M, Basciani S. Treatment with myo-inositol and selenium ensures euthyroidism in patients with autoimmune thyroiditis. International Journal of Endocrinology (2017)
Zhang J et al.. Vitamin D supplementation for autoimmune thyroiditis: a systematic review and meta-analysis. International Journal of Endocrinology (2021)
Mahmoodianfard S et al.. The effects of zinc and selenium co-supplementation on thyroid function in overweight and obese hypothyroid female patients: a randomized double-blind controlled trial. Journal of the American College of Nutrition (2015)
Biotin interference in diagnostic immunoassays and thyroid function tests: FDA safety communication. US Food and Drug Administration (2019)View source
Selenium: Fact Sheet for Health Professionals. NIH Office of Dietary SupplementsView source
Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary SupplementsView source
Jabbar A et al.. Vitamin B12 deficiency common in primary hypothyroidism. Journal of the Pakistan Medical Association (2008)
Spaggiari G et al.. Probiotics and thyroid function: a randomized trial of probiotic supplementation in hypothyroid patients on levothyroxine. Frontiers in Endocrinology (2017)
Underactive thyroid (hypothyroidism) - Treatment. NHSView source

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This guide is educational and is not medical advice. Herbs and supplements can interact with prescription medicines and are not regulated like drugs. Talk with a clinician or pharmacist before starting anything new, especially if you are pregnant, breastfeeding, have a chronic condition, or take medication.