I’m always on a mission to find the most efficient ways to improve thyroid function and calm the autoimmune response for better long term health outcomes in Hashimoto’s. Because who wants to get progressively worse with each passing decade? Not moi! Even if you have had Hashimoto’s and been reliant on medication for many years and remission is not likely for you, you can still slow progression of tissue destruction which is a worthwhile endeavour, don’t you agree?

This blog has existed as a series of notes that I’ve been adding to every time I find more juicy research for the better part of a year. I’ve been slowly working on it to bring you some helpful info that I believe can seriously move the dial on pathology results and symptoms for those of us with Hashimoto’s or Hypothyroidism.

I’m guessing you’ve probably heard of selenium but haven’t given it much thought, which if you have Hashimoto’s or Hypothyroidism, may be a mistake. Selenium is an essential mineral, which means our body cannot produce it, so we need to get it from our dietary intake. It is also a trace mineral, meaning we only need very small amounts of it compared to macro minerals such as calcium or magnesium. However, its small dose does not define how mighty this mineral really is. If adequate selenium is ingested our body produces selenoproteins which are very powerful antioxidants that are important for thyroid health and function.

Selenium has an important cellular health job description, it is essential in adequate amounts for the following:

• Antioxidant activity
• Immune function
• Cellular protection of cardiovascular & neurological systems
• Male & female reproductive health
• Foetal development
• DNA synthesis
• Liver detoxification
• Gall bladder health & function
• Thyroid function & cellular protection

Let’s explore selenium in more depth to see if adding it to your health regime might be helpful for your long term thyroid health outcome.

The Selenium-Thyroid Connection

CONVERSION

Perhaps the most important point I can make in this blog post is that without adequate selenium levels, you will have under-conversion of thyroid hormones T4 to T3. T3 is the active thyroid hormone that travels around the body doing all the heavy lifting in your metabolism and virtually every system and most cells in your body, helping you feel less zombie, more yourself. This is because deiodinase enzymes are responsible for this all important conversion and this enzyme in selenium dependant.

Inadequate selenium -> inadequate deiodinase enzymes -> inadequate thyroid hormone conversion -> inadequate T3 -> thyroid hormone deficiency -> low energy, weight gain, brain fog and all the other fun consequences of hypothyroidism.

After working with many women with Hashimoto’s disease I believe optimising conversion to increase T3 is the key to helping them feel better. Current treatment focuses on increasing T4 hormone levels with thyroid hormone replacement (levothyroxine) but what if it’s not being efficiently converted? Food for thought.

How does conversion work? Deiodinasation of T4 removes one iodine molecule to convert it into T3, the more active form of thyroid hormone. This occurs in the gut, liver, kidneys and some in the thyroid itself. If there is under conversion of thyroid hormones you might still be experiencing hypothyroid symptoms even if your pathology results look normal with adequate T4 levels. You can read more about thyroid conversion in my blog Optimising Thyroid Conversion.

PROTECTION

The thyroid is where you’ll find the highest concentration of selenium in the human body, for good reason, it has an important job to do. When thyroid hormones are produced natural byproducts that cause oxidative stress like hydrogen peroxide are generated. In a healthy body with adequate antioxidants and biochemical defences (like sufficient selenium levels and a diet that features an abundance of antioxidant rich plant foods) this is counteracted and self-corrected. If not, oxidative damage can escalate, damaging the cells of thyroid tissue and potentially contribute to the onset of Hashimoto’s or indeed Grave’s disease.

In a 2024 meta-analysis of 35 clinical studies, supplementation of selenium for a period that varied between 3-6 months depending on the study, was associated with an average reduction of 5-10% in TSH¹. This effect was only seen in those who were in the early stages of Hashimoto’s disease and were not yet taking thyroid hormone replacement. Another 2025 meta analysis supports these findings, suggesting that 6 months of selenium supplementation were needed to see any significant reduction in TSH². While these modest results are still beneficial, It is with thyroid antibodies and autoimmunity that selenium demonstrates more promising results.

Selenium & Autoimmunity

LOWER ANTIBODIES

Research has shown that selenium supplementation can reduce thyroid antibodies associated with Hashimoto’s disease, notedly TPOAb, not TGAb, and improve patient mood and lifestyle satisfaction scores³. The same analysis as above found a more pronounced effect on thyroid peroxidase antibodies (TPOAb), a key marker of autoimmune activity. Individual trials using selenium supplements reported average reductions in TPOAb of around 10–21% over 3–6 months, with some studies showing greater reductions in people with higher baseline antibody levels. One study demonstrated a 21% reduction in TPOAb over 12 months⁴. Selenium was also associated with a significant reduction in oxidative stress markers, suggesting a protective, anti-inflammatory effect within thyroid tissue⁵ which makes me wonder what outcomes could be achieved with a longer time frame like 5 or even 10 years? Sadly, no studies covered anywhere near this length of time.

Selenium has been identified as one of the common nutrient deficiencies in those with autoimmunity, including Hashimoto’s disease. This is something I have observed time and again with my client’s pathology results. As a result it has been hypothesised that selenium deficiency may form part of the aetiology or cause of Hashimoto’s thyroiditis. We know that selenium protects the thyroid from cellular damage caused by thyroid hormone production so is it possible that being deficient causes enough inflammation to trigger an autoimmune response if other environmental triggers are present? Quite possibly. Would it be the only contributing factor? Doubtful.

Importantly, selenium supplementation did not significantly change thyroid hormone levels T3 or T4, TSH, thyroid size, or eliminate the need for medication where it was required. So selenium is definitely not a Hashimoto’s miracle cure or thyroid hormone substitute, but it is a supportive nutrient that may help calm autoimmune activity and reduce thyroid stress and damage.

Causes of Suboptimal and/or Selenium Deficiency

I won’t get into Australian soil and modern farming methods again – wellllllll, maybe I will – Australian soil is very old and it’s not very fertile or suitable for growing food. Indigenous Australians were very resourceful foragers and regenerative land caretakers. When Europeans settled in Australia they brought their farming techniques with them but Australian soil was not and is still not well suited to mass mono crops on repeat. Australia soil is old and weathered with widespread nutrient depletion. Minerals like selenium have been leached and washed away and so we are not consuming as much of these macro or micro minerals through food grown or pastured on Australia soils. Fresh produce and grains have been modified over the past few decades for a larger yield and bigger produce but as a result of these methods, we lose nutrient density. That is on top of the soil nutrient deficiency. This leaves us vulnerable to suboptimal levels and/or mineral deficiencies. Therefore selenium status varies across the world. What is true for many living in Australia, where I now sit and type this blog, may not be true for you and your location.

The SAD (Standard Australia Diet) is alarmingly high in ultra processed foods which do not provide essential daily nutrients. There are some cases where minerals such as calcium or iron are fortified into these processed foods. However, these forms of minerals are often cheap, poorly absorbed and cannot in any way replace a diet rich in a variety of whole foods. The SAD diet does not provide enough daily selenium.

Vegetarians and vegans are at a higher risk for deficiency of selenium as food with highest selenium content are animal based, with the exception of Brazil nuts.

Smoking is also a risk factor as it depletes selenium levels. Selenium reserves are utilised and depleted in their capacity as antioxidants to combat the high levels of oxidative stress caused by smoking as well as the high levels of cadmium and other heavy metals found in cigarettes. High oxidative stress, regardless of its source will deplete antioxidants like selenium so vaping is proving to be another source of selenium depletion too.

Selenium Pathology

Selenium is tested via blood test. The range for selenium is extremely narrow, it varies but most Australian lab ranges for selenium at 0.8-2.0. In my experience you want to be at the upper range or slightly above to see benefits. Selenium lab ranges are currently under review in Australia and I wouldn’t be surprised if they raise the upper limit for serum selenium levels.

TOXICITY SYMPTOMS

In 2008 in the USA a supplement company released a liquid selenium that was accidentally 200x the advertised dose⁶. Toxicity symptoms were hair loss, brittle nails and nail loss, diarrhoea, nausea, joint pain, fatigue and foul breath. This is a cautionary tale of buying cheap and unregulated US supplements off iHerb. While it is extremely unlikely that you could ingest doses this large, especially if you are taking a TGA approved product from Australia, it’s good to know that early toxicity symptoms include gastrointestinal upset and nausea, garlic breath, a metallic taste in the mouth and hair loss. Most products and supplements in Australia warn not to exceed 150mcg per day of selenium. I’ve found doses of 150-200mcg to be safe and effective, however, every body is unique and responds differently which is why regular screening is important. If you have self-prescribed a few supplements you need to be very cautious that you aren’t taking a little selenium here and little there, because it can add up. You also need to avoid having a lot of Brazil nuts as they can be very high in selenium too. For the record, I don’t recommend self-prescribing supplements for this reason. I see a practitioner for my own health and you should too, don’t DIY supplements. Tiktok doesn’t count.

Selenium & iodine

Selenium and iodine work synergistically to support thyroid function and health. They need to be assessed and are often supplemented together. Iodine, like selenium, is a micronutrient and trace mineral that is crucial for thyroid function. It is used as the building blocks of T4 thyroid hormone itself. Before trialling a selenium supplement it is recommended that both selenium and iodine status are assessed via a blood test. If you are also iodine deficient then it’s important you take selenium alongside iodine to offset any oxidative damage the iodine utilisation may cause in the thyroid gland itself. If iodine stores are adequate you can take selenium alone but it is not recommended to take iodine alone without selenium.

Food Sources of Selenium

In order, the highest sources of selenium are: brazil nuts, all shellfish but particularly oysters, seafood, organ meats, muscle meat, dairy and whole grains but the level of selenium found in plant foods varies according to geographical location and soil selenium levels.

Anything that gives you an excuse to eat more oysters is fine by me!

If you are going to take the Brazil nut approach I would start with a blood test, 1-2 Brazil nuts per day and a follow up test in three months, increase or decrease by a single nut and test after another three months until you hit the right maintenance ‘dose’. Be aware that the levels of selenium will vary greatly depending on where the brazil nut is grown so find a source and stick with that country of origin for more consistent results.

Selenium Supplementation

While I do generally recommend a food first approach, when it comes to sensitive autoimmune clients with chronic inflammatory conditions and higher health stakes, I find it harder to justify. As virtuous as a food first approach sounds, the truth is that selenium is one of the easiest nutrients to overdose and reach toxicity with, yes, even with food sources. The lab ranges are extremely narrow. And iodine in excess levels can be disastrous for Hashimoto’s as it upregulates antibody production and worsens disease progression. Consuming large amounts from food sources of iodine or selenium may put you at risk of toxicity or may simply be more difficult to quantify, making treating deficiencies with food alone difficult. Brazil nuts for example can vary widely in their selenium content but as few as 4 per day could cause toxic levels of selenium in your system. Professional assessment of your pathology and specific dosing and reviews are necessary before you can maintain levels with diet alone. If supplementing, or indeed trying to reach therapeutic levels with food alone, iodine and selenium need to be monitored closely. I recommend quarterly testing initially and then every six months for maintenance.

it’s worth noting that in one of the studies mentioned above, after 12 months, participants ceased taking selenium and the 21% reduction in TPOAb was reduced by 4.8%, indicating that it may need to be taken long term to maintain adequate selenium status.

The good news is that several reputable, TGA approved, Australian companies manufacture selenium in a dropper which you can add to water very easily. It has almost no taste and is not expensive.

Final Thoughts

While we could do with a large scale clinical trial, several meta analyses of many smaller trials show promising results with improving selenium status in people with Hashimoto’s thyroid disease. Higher mood and life satisfaction scores, modest improvements in TSH and potentially significant reductions in TPOAb antibody activity make selenium a mineral that is worth considering for your long term thyroid health regime. Selenium supplementation is not without risk though. It’s important to stick to moderate dosing and commit to regular testing. Like iodine, selenium is not a supplement to DIY.

Me? I take a few drops everyday in a glass of water with breakfast in a low to moderate dose and check it annually via blood test to ensure levels are stable. They have been for several years which is why I have been able to draw out my pathology to 12 months instead of 3-6 months. I do eat ABC (almond, brazil, cashew) nut butter a couple of times per week but not in great quantities. And if I ever have the means to enjoy oysters more than once in a blue moon I’ll gladly reassess my selenium dose!

If you want to learn more about optimal pathology levels in Hashimoto’s there is a guide included in my program Food Freedom, Hashimoto’s Edition.

1. Winther KH, Wichman JEM, Bonnema SJ, Hegedüs L. Selenium supplementation for Hashimoto’s thyroiditis: a systematic review and meta-analysis. Thyroid. 2024. ↩︎
2. Zhang, Heng MMa; Yang, Yunkai MDb; Liu, Shaohua MMc; Yang, Yang MDd; Liu, Zhelong MDc,*. Clinical efficacy of selenium supplementation in patients with Hashimoto thyroiditis: A systematic review and meta-analysis. Medicine 104(35):p e44043, August 29, 2025. ↩︎
3. Toulis KA, Anastasilakis AD, Tzellos TG, et al. Selenium supplementation in the treatment of Hashimoto’s thyroiditis: a systematic review and meta-analysis. Thyroid. 2010;20(10):1163–1173. ↩︎
4. Mazokopakis EE, Papadakis JA. Effects of L-selenomethionine supplementation on serum anti-TPO levels in patients with Hashimoto’s thyroiditis. Thyroid. 2007;17(7):609–612. ↩︎
5. Winther KH, Wichman JEM, Bonnema SJ, Hegedüs L. Selenium supplementation for Hashimoto’s thyroiditis: a systematic review and meta-analysis. Thyroid. 2024. ↩︎
6. MacFarquhar JK, Broussard DL, Melstrom P, Hutchinson R, Wolkin A, Martin C, Burk RF, Dunn JR, Green AL, Hammond R, Schaffner W, Jones TF. Acute selenium toxicity associated with a dietary supplement. Arch Intern Med. 2010 Feb 8;170(3):256-61. ↩︎