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Supplements for Hashimoto's: Selenium, Vitamin D, Iodine and What to Avoid

For Hashimoto's, no supplement replaces thyroid hormone when your thyroid is underactive. Selenium lowers thyroid antibodies a little (grade B), but in a year-long trial of 412 people on levothyroxine, people felt no better on it than on placebo. Vitamin D lowered antibodies in small trials (grade C). Avoid kelp and iodine pills, which can worsen thyroid disease, and take calcium or iron 4 hours apart from levothyroxine.

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Hashimoto's is the most common cause of an underactive thyroid in places where people get enough iodine (Huwiler 2024). Here is what the trials show for selenium, vitamin D, myo-inositol, gluten-free eating and ashwagandha, why kelp and iodine pills can backfire, and how supplements can throw off both your thyroid medicine and your blood tests.

The short version

  • Selenium lowers thyroid antibodies a little, but did not make people on thyroid medicine feel better than placebo. Grade B (Huwiler 2024; Larsen 2024).
  • Vitamin D lowered antibodies in small trials; a second analysis disagreed. Grade C (Tang 2023; Peng 2024).
  • Kelp and iodine pills can bring on or worsen an underactive thyroid in people with Hashimoto's. Grade C (Bürgi 2010; NIDDK).
  • Calcium, iron, coffee, soy and fiber can cut how much levothyroxine you absorb, and biotin can falsify thyroid blood tests. Grade C: the evidence is limited, but the advice to space them out is cautious (Wiesner 2021; Ylli 2021).

What helps Hashimoto's most?

Thyroid hormone, when your thyroid is underactive. Hashimoto's is an autoimmune disease: the immune system slowly damages the thyroid, and most people with it develop hypothyroidism (an underactive thyroid). Levothyroxine, a pill identical to the body's own thyroxine (T4), is the recommended treatment; if your levels are normal, your doctor may just check them over time (NIDDK). NIDDK adds that researchers are studying vitamin D and selenium, but no specific guidance on them exists yet. The dose of thyroid medicine is your prescriber's call, based on blood tests; this guide gives none.

Does selenium help Hashimoto's?

Grade B that it lowers antibodies, and grade B that this did not translate into feeling better. In a 2024 review of trials, selenium lowered thyroid peroxidase antibodies (TPOAb, the main Hashimoto's antibody) across 29 groups of 2,358 people, and lowered TSH slightly in the 7 groups (869 people) not taking thyroid hormone; side effects were no more common than on control (Huwiler 2024). But in the largest trial, 412 adults on levothyroxine took 200 mcg a day as selenium yeast or a placebo for a year. Quality of life improved by the same amount in both groups, and their levothyroxine needs did not change, even though antibodies ended a little lower on selenium (Larsen 2024). Lower antibodies are a lab result, not a symptom: in the trials we found, the drop did not bring better quality of life or a lower medicine dose.

More is not better. In a trial of 1,202 people followed for an average of 7.7 years, 200 mcg of selenium a day was tied to about half again as many new cases of type 2 diabetes (58 vs 39) as placebo (Stranges 2007). Upper limit for adults: 400 mcg of selenium a day (Europe sets 255 mcg), because more can make hair and nails brittle or fall out; take more only with blood tests or your clinician's advice (NIH ODS; check your stack). Our selenium question page has the full trial detail. Our database grades selenium D for Hashimoto's from a single study; we give B because the pooled antibody result is moderate in certainty.

Does vitamin D help Hashimoto's?

Grade C, for lab markers only. In a 2023 analysis of 12 trials (862 people with Hashimoto's), vitamin D lowered both thyroid antibodies and nudged TSH and thyroid hormone levels in a healthier direction, more so after 12 weeks (Tang 2023). A 2024 network analysis found that vitamin D alone did not significantly lower antibodies over 6 months, while selenium did (Peng 2024). Neither measured how people felt. If a blood test shows you are low, correcting it is reasonable on its own merits. Upper limit for adults: 4,000 IU of vitamin D a day, because more can raise blood calcium too high; take more only with blood tests or your clinician's advice (NIH ODS). See our vitamin D page.

Do myo-inositol and selenium together help?

Grade C. In one 6-month trial in 168 people with Hashimoto's and mildly high TSH (3 to 6), myo-inositol plus selenium lowered TSH and antibodies, and raised T4, more than selenium alone (Nordio 2017). The 2024 network analysis did not find a significant antibody drop for the pair (Peng 2024). One trial with blood-test outcomes is not enough to rely on. See our inositol page.

Should you take iodine or kelp with Hashimoto's?

No. Grade C for the harm, from biology and case reports. NIDDK says people with Hashimoto's may be sensitive to iodine's side effects: large amounts from kelp, dulse or other seaweed, iodine-rich medicines or iodine supplements may cause an underactive thyroid or make it worse (NIDDK). In autoimmune thyroiditis, the thyroid's iodine pump can fail to shut down when flooded, and long-term hypothyroidism follows (Bürgi 2010). A 2026 review warns that "iodine reset" and kelp products promoted online can deliver iodine doses far above recommended intakes (Taylor 2026). Excess iodine can push the other way too: a 70-year-old woman developed an overactive thyroid, with a racing heart and weight loss, 3 months after starting kelp slimming tablets (Gherbon 2019). Pregnancy changes iodine needs; ask your doctor rather than cutting it out. See our iodine page.

Does a gluten-free diet help Hashimoto's?

Grade C, unclear. Without celiac disease, only 3 small trials (110 people) have tested it. Pooled, the effect on thyroid hormones was very uncertain, one antibody fell while the other rose, and the trials had serious design problems (Araújo 2025). If you have celiac symptoms, get tested before changing your diet.

Is ashwagandha safe with Hashimoto's?

We'd avoid it. NCCIH says ashwagandha is not recommended for people with autoimmune or thyroid disorders and may interact with thyroid hormone medicines (NCCIH). In an 8-week trial of 50 adults with mildly underactive thyroids, 600 mg a day of root extract shifted TSH, T3 and T4 significantly compared with placebo (Sharma 2018). A supplement that moves thyroid levels makes your medicine dose harder to set. See can I take ashwagandha with levothyroxine?.

What we'd skip

  • Kelp, seaweed tablets and "iodine reset" products. They can cause or worsen hypothyroidism, or trigger an overactive thyroid (NIDDK; Gherbon 2019).
  • Ashwagandha with any thyroid disorder (NCCIH).
  • Selenium above the upper limit, or for years without a reason. The antibody drop did not improve how people felt, and a long trial tied it to more diabetes (Larsen 2024; Stranges 2007).

Safety and interactions

  • Spacing from levothyroxine. Calcium and iron supplements, coffee, soy and fiber all lowered levothyroxine absorption in studies (Wiesner 2021); NIDDK names grapefruit juice, espresso, soy and multivitamins with iron or calcium (NIDDK). Take calcium, iron or magnesium 4 hours apart from your thyroid pill: see can I take iron, calcium or magnesium with levothyroxine?.
  • Biotin and thyroid blood tests. When 13 adults took 10 mg of biotin daily for 8 days, some lab tests gave false TSH, T4 and thyroglobulin results, worst 2 hours after a dose (Ylli 2021). Read does biotin affect thyroid and troponin tests? before your next blood draw, and tell the lab what you take.
  • Several supplements at once: check your stack against the upper limits.
  • Don't change your levothyroxine dose on your own; ask the prescriber or pharmacist. Pregnancy or breastfeeding: ask a clinician.

When to see a doctor

  • Symptoms of an underactive thyroid: tiredness, weight gain, feeling cold, joint and muscle aches, constipation, dry skin or thinning hair, heavy or irregular periods, or a slow heartbeat (NIDDK; MedlinePlus).
  • A racing heart, shakiness, poor sleep or weight loss after starting kelp, iodine or a new supplement.
  • Planning a pregnancy or pregnant with Hashimoto's: thyroid needs change, so ask your doctor.
  • Severe drowsiness or confusion with known hypothyroidism is an emergency: severe untreated hypothyroidism can lead to coma (MedlinePlus).

FAQ

Does selenium help Hashimoto's? It lowers thyroid antibodies a little (grade B), but in a year-long trial of 412 people on levothyroxine, quality of life improved no more than on placebo.

What supplements should I avoid with Hashimoto's? Kelp, seaweed tablets and iodine pills, which can bring on or worsen an underactive thyroid, and ashwagandha, which NCCIH says is not recommended with thyroid disorders.

Does vitamin D help Hashimoto's? It lowered thyroid antibodies in pooled small trials, but a second analysis disagreed and no trial measured how people felt (grade C). Correcting a measured deficiency is reasonable.

Does a gluten-free diet help Hashimoto's? It is unclear (grade C): only 3 small trials in people without celiac disease, with mixed antibody results.

How long should I wait between levothyroxine and supplements? Take calcium, iron or magnesium 4 hours apart from levothyroxine. Coffee, soy and fiber can also lower how much you absorb.

Sources

  1. Huwiler VV, et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024. PubMed 38243784 · doi:10.1089/thy.2023.0556
  2. Larsen C, et al. Selenium supplementation and placebo are equally effective in improving quality of life in patients with hypothyroidism. Eur Thyroid J. 2024. PubMed 38215286 · doi:10.1530/ETJ-23-0175
  3. Tang J, et al. Effects of vitamin D supplementation on autoantibodies and thyroid function in patients with Hashimoto's thyroiditis: A systematic review and meta-analysis. Medicine (Baltimore). 2023. PubMed 38206745 · doi:10.1097/MD.0000000000036759
  4. Peng B, et al. Effects of different supplements on Hashimoto's thyroiditis: a systematic review and network meta-analysis. Front Endocrinol (Lausanne). 2024. PubMed 39698034 · doi:10.3389/fendo.2024.1445878
  5. Bürgi H. Iodine excess. Best Pract Res Clin Endocrinol Metab. 2010. PubMed 20172475 · doi:10.1016/j.beem.2009.08.010
  6. Wiesner A, et al. Levothyroxine Interactions with Food and Dietary Supplements-A Systematic Review. Pharmaceuticals (Basel). 2021. PubMed 33801406 · doi:10.3390/ph14030206
  7. Ylli D, et al. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin. Thyroid. 2021. PubMed 34042535 · doi:10.1089/thy.2020.0866
  8. Stranges S, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007. PubMed 17620655 · doi:10.7326/0003-4819-147-4-200708210-00175
  9. Nordio M, et al. Myo-inositol plus selenium supplementation restores euthyroid state in Hashimoto's patients with subclinical hypothyroidism. Eur Rev Med Pharmacol Sci. 2017. PubMed 28724185
  10. Taylor PN, et al. What Endocrinologists Should Know About Iodine: Population Deficiency, Individual Excess and Misinformation in the United Kingdom. Clin Endocrinol (Oxf). 2026. PubMed 42144726 · doi:10.1111/cen.70162
  11. Gherbon A, et al. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report. Medicine (Baltimore). 2019. PubMed 31517826 · doi:10.1097/MD.0000000000017058
  12. Araújo EMQ, et al. Effects of Gluten-Free Diet in Non-Celiac Hashimoto's Thyroiditis: A Systematic Review and Meta-Analysis. Nutrients. 2025. PubMed 41228508 · doi:10.3390/nu17213437
  13. 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. PubMed 28829155 · doi:10.1089/acm.2017.0183