Selenium and medications.
Selenium is in the Distil supplement database, evidence Grade A. The page below lists every medication we have explicitly assessed it against.
Selenium is a trace mineral the body needs in small amounts, and UK soils are low in it, so moderate shortfall is common here. Its clearest job is helping convert thyroid hormone T4 into the active T3, which makes it especially relevant in hypothyroidism and Hashimoto's, and it also drives glutathione peroxidase, one of the body's antioxidant enzymes. The grade depends on where you are starting from. Correcting a documented selenium shortfall is Grade A, and that is what the strong evidence covers. If your selenium is already adequate, the evidence that supplementing further improves thyroid function is Grade C: in Hashimoto's, the group most likely to benefit, pooled trials lower TSH slightly and lower thyroid peroxidase antibodies, but do not change free T4, free T3, thyroglobulin antibodies or thyroid volume. Antibody titre is a marker rather than a symptom or an outcome, which is why it does not lift the grade on its own. Immune function is Grade C. Male fertility is Grade C, and the reason is worth stating plainly. Two pooled analyses we hold report better sperm concentration, motility and shape, while the network analysis of the same trial pool finds selenium significant on none of those three. The measure that matters more is pregnancy, and where it has been pooled across selenium, carnitine and CoQ10 together (Sharma 2021), pregnancy rates were 16.2 per cent on treatment against 11.2 per cent on placebo, which did not reach significance. So the honest reading of what we hold is that selenium may move your semen numbers, and that in our records nothing yet shows it raises the chance of a pregnancy. Rayman 2012 remains the standing review, and it is worth noting it advises against supplementing when you are already replete. Selenomethionine at 55 to 200mcg is the usual supplemental range; the upper safe limit is 400mcg a day and there is no reason to push past 200mcg from supplements. Going too high causes selenosis: hair loss, brittle nails and a garlic odour on the breath. On interactions, selenium pairs with iodine for thyroid work since both are needed, and anyone on anticoagulants should have it monitored. At sensible doses it is well tolerated, so more is not better here.
Below are the 1 documented pair we have explicitly assessed for Selenium: 1 green. The pairs cluster around 1 mechanism: Thyroid cofactor (beneficial). Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not listed here is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by medication, and the missing-item form at the bottom of the page routes any uncatalogued medication into our next curation pass.
Documented interactions
Last reviewed 13 June 2026. Each interaction below lists the evidence its assessment drew on: some rest on studies of the exact pair, others on mechanism evidence, where a citation may support one part of the reasoning rather than the exact combination. The evidence line on each entry says which.
Thyroid cofactor (beneficial)
Green Does Selenium interact with Levothyroxine?
Selenium and levothyroxine work together rather than against each other. Selenium is a building block the body needs to convert thyroid hormone into its active form, and in people with Hashimoto's it can lower thyroid antibodies. That antibody fall is a blood-test change, and trials have not shown it shifts your thyroid hormone levels or alters the course of the condition. Whether it helps people feel better is genuinely unsettled: one review of people already taking levothyroxine did report better mood and general wellbeing, though it rests on three small trials, while a trial in women who had the antibodies but normal thyroid function, and were not on levothyroxine, found no change in quality of life. It does not change how much levothyroxine you absorb or how much you need, and good trials found no safety problem combining them. Keep your selenium to around 100 to 200 micrograms a day, since very high doses are harmful in their own right. We treat this pair as safe to combine.
Evidence: Supported by human study data.
What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope. Use the checker below to surface any medication, and submit a missing item if you take something we have not catalogued.
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How we grade severity, choose what's in scope, and what we exclude.
Every call on this page is reasoned. We publish the full rubric for severity tiers, the medication inclusion logic, the evidence grades we accept, and what we deliberately leave out. About three thousand words. Worth reading once if you use this tool more than occasionally.
Read the full methodologyA full supplement review, built around you, not a generic stack.
The free checker answers one interaction question at a time. A Distil report reviews the bigger picture: your goals, diet, medications, current supplements and any blood results you provide, then shows what appears worth keeping, changing, adding or leaving out.
- what your diet already covers, so you are not adding a capsule for something food is already handling
- which evidence-graded compounds are worth considering for your goals, and which were considered but left out
- what to keep, change or stop from what you already take, plus the order to introduce anything new
- how your medications and complete supplement stack fit together, with daily timing and a clear point for reassessment
The interaction check is one section of the full report. You also get the reasoning behind each recommendation and the evidence references used.
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