Selenium, vitamin D, and iron have the strongest evidence for supporting thyroid function in people with Hashimoto’s thyroiditis. Several other nutrients play supporting roles, and a few popular supplements can actually make the condition worse. Here’s what the research shows for each one.
Selenium
Selenium is the most studied supplement for Hashimoto’s, and for good reason. Your thyroid contains more selenium per gram of tissue than any other organ. It protects thyroid cells from oxidative damage during hormone production and is essential for the enzymes that convert thyroid hormones into their active form.
The form that matters is selenomethionine, typically at 200 micrograms per day. In clinical trials, this dose significantly reduced TPO antibodies, the primary marker of autoimmune attack on the thyroid. One study of 169 participants found a mean antibody reduction of 345 IU/mL compared to placebo. When selenomethionine was combined with thyroid hormone medication, the reduction was even more pronounced, reaching over 1,500 U/mL in one trial. Sodium selenite, another common supplement form, did not produce the same results, so check the label.
The recommended daily intake for adults is only 55 micrograms, meaning 200 micrograms is a therapeutic dose well above baseline needs. Brazil nuts are the richest food source, but their selenium content varies wildly depending on where they were grown. A supplement gives you a consistent dose. Selenium toxicity is possible above 400 micrograms per day, so more is not better here.
Vitamin D
Low vitamin D is extremely common in people with Hashimoto’s, and correcting it appears to calm the autoimmune response. In a series of studies from Poland, patients who supplemented with 2,000 to 4,000 IU of vitamin D daily for six months saw significant decreases in both TPO and thyroglobulin antibodies. The key threshold seems to be reaching a blood level of at least 30 ng/mL, which many Hashimoto’s patients fall below.
Getting your 25(OH)D level tested is a practical first step. If you’re well below 30 ng/mL, a higher initial dose may be appropriate, tapering down once you reach the target range. Vitamin D is fat-soluble, so taking it with a meal that contains some fat improves absorption.
Iron and Ferritin
Iron does something specific in Hashimoto’s that other nutrients don’t: it powers thyroid peroxidase (TPO), the same enzyme the immune system is attacking. TPO is a heme-containing enzyme, meaning it literally needs iron to function. Without adequate iron, your thyroid can’t efficiently attach iodine to its hormone precursors, which is the first step in making thyroid hormones.
Research shows that people with ferritin levels below 20 ng/mL have significantly higher TSH and lower free T4 compared to those above that threshold. A large Spanish population study found that ferritin below 30 ng/mL was associated with lower levels of both T4 and T3. Many practitioners now aim for ferritin well above the bare minimum lab range, particularly in Hashimoto’s patients who remain symptomatic despite adequate thyroid medication.
Iron supplements should be taken separately from thyroid medication by at least four hours, as iron blocks absorption of levothyroxine. If your ferritin is genuinely low, supplementing can meaningfully improve how well your thyroid medication works.
Myo-Inositol Plus Selenium
One of the more interesting findings in recent years involves combining myo-inositol with selenium. In a study of autoimmune thyroiditis patients, six months of this combination reduced TSH from 4.32 to 3.12 mIU/L and significantly lowered both types of thyroid antibodies: TPO antibodies dropped from about 721 to 620 IU/mL, and thyroglobulin antibodies fell from 345 to 289 IU/mL. Notably, selenium alone typically only reduces TPO antibodies, while the combination brought down both.
Myo-inositol is a naturally occurring sugar alcohol involved in cellular signaling, including the signaling pathway that TSH uses to communicate with thyroid cells. This combination is widely available as a single supplement in Europe and increasingly in the U.S.
Zinc and Magnesium
Both zinc and magnesium support the enzymes that convert T4 (the inactive storage form of thyroid hormone) into T3 (the active form your cells use). Research on zinc-deficient patients found that supplementation improved T4-to-T3 conversion. If you’re taking levothyroxine but still feel hypothyroid, poor conversion is one possible explanation, and zinc or magnesium deficiency could be a contributing factor.
Zinc deficiency isn’t rare. It’s common in people who eat little red meat, in vegetarians, and in those with gut absorption issues, all of which overlap significantly with the Hashimoto’s population. Magnesium deficiency is even more widespread in the general population. Neither nutrient needs to be taken in high doses; staying within the standard supplemental range (15 to 30 mg for zinc, 200 to 400 mg for magnesium) is typically sufficient.
Vitamin B12
About 27% of people with autoimmune thyroid disease are deficient in vitamin B12, a rate far higher than the general population. The connection is another autoimmune overlap: roughly 27% of people with autoimmune thyroid disease also carry antibodies against the stomach’s parietal cells. These cells produce intrinsic factor, the protein your gut needs to absorb B12. When the immune system damages them, B12 absorption drops regardless of dietary intake.
Symptoms of B12 deficiency, including fatigue, brain fog, numbness, and mood changes, overlap heavily with hypothyroid symptoms. If you’ve optimized your thyroid levels and still feel off, B12 is worth checking. Sublingual or methylated forms bypass the gut absorption issue that parietal cell antibodies create.
Omega-3 Fatty Acids
Hashimoto’s is fundamentally an inflammatory condition, and omega-3 fatty acids from fish oil help regulate that inflammation. Recent research found significant correlations between CRP levels (a standard inflammation marker) and anti-inflammatory compounds derived from EPA and DHA in Hashimoto’s patients. These omega-3 derivatives, called resolvins and protectins, actively help resolve inflammation rather than just blocking it. One derivative also correlated with free T4 levels, suggesting a direct link between omega-3 metabolism and thyroid hormone production.
This doesn’t mean fish oil will lower your antibodies on its own, but managing systemic inflammation is a meaningful part of living with any autoimmune condition.
Probiotics and Gut Health
The gut-thyroid connection is real but the supplement evidence is still limited. A meta-analysis of eight randomized controlled trials found that probiotics and prebiotics did not significantly change TSH, free T4, or free T3 levels. They did reduce one specific antibody (TRAb), but that marker is more relevant to Graves’ disease than Hashimoto’s.
That said, gut health matters for Hashimoto’s in indirect ways. Intestinal permeability issues can drive autoimmune activity, and a healthy microbiome supports nutrient absorption, which circles back to all the deficiencies discussed above. Multi-strain formulas containing Lactobacillus and Bifidobacterium species have been used in most of the existing trials. Probiotics are unlikely to hurt, but they shouldn’t be your first priority over the nutrients with stronger evidence.
Supplements to Be Cautious About
Iodine
This is the big one. Many thyroid support formulas contain iodine because it’s essential for making thyroid hormones. But in Hashimoto’s, excess iodine can pour fuel on the autoimmune fire. Animal studies show that iodine-excessive diets increase thyroid damage and lymphocyte infiltration in a dose-dependent manner. In a human study, Hashimoto’s patients taking just 250 micrograms of supplemental iodine daily had significantly higher rates of thyroid dysfunction than controls. Unless you have a confirmed iodine deficiency, supplemental iodine is best avoided. Check your multivitamin and any “thyroid support” blends for hidden iodine content.
Biotin
Biotin doesn’t harm your thyroid, but it can make your lab results look wildly inaccurate. High-dose biotin supplements (commonly taken for hair and nails, which many Hashimoto’s patients use) interfere with the technology used in thyroid blood tests. This can produce falsely low TSH or falsely high free T4 readings, potentially leading to incorrect medication adjustments. The American Thyroid Association recommends stopping biotin at least two days before any thyroid function test. If you take biotin for hair loss, just pause it before your next blood draw.