Distil Interaction checker
Personal supplement reports
Supplement · Grade A

Selenium and medications.

Every documented pair, every citation. Below: 1 documented pair grouped by mechanism.

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. The body uses it to make glutathione peroxidase, one of its antioxidant enzymes, and to convert thyroid hormone T4 into the active T3. 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, we do not grade it for thyroid function: in Hashimoto's, pooled trials lower thyroid peroxidase antibodies but do not change free T4 or free T3, and antibody titre is a marker rather than a symptom or an outcome. 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 and motility, and one of them better shape, while a later network analysis of 18 trials 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 European Food Safety Authority's upper safe limit, set in 2023, is 255mcg a day from food and supplements together, so a 200mcg supplement leaves little room once your diet is counted, and there is no reason to go higher. Taking too much over time can cause selenosis, whose signs include hair loss, brittle nails and a garlic smell on the breath. Selenium has not been studied alongside blood thinners: one small trial found it reduced platelet clumping and another found no change, so the concern is theoretical, and if you take an anticoagulant it is worth a mention to your GP or pharmacist. 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 good to know. 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)

Good to know 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.

PMID 11932302 · PMID 20883174 · PMID 38243784 · PMID 23844613 · BNF: Levothyroxine

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.

Database last updated 2026-11-13. Free for everyone. No account, no paywall. · Browse coverage A-Z

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For adults over 18. This tool gives evidence-graded information, not medical advice. Always discuss changes with your GP or pharmacist, especially if you take any medication, are pregnant, breastfeeding, or have a serious health condition.
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How the checker decides

Every result has a source

Each pair we have assessed is cited to a published study (PMID) or the British National Formulary, so you can read the source yourself.

Four kinds of answer

Severe, caution, no known interaction, and not yet assessed. A pair we have not assessed is never shown as clear.

  • Severe
  • Caution
  • No known interaction
  • Not yet assessed

Nothing in our data is not a clearance

When nothing in our data records a problem with a pair, the result says no known interaction at typical doses, which is not a clearance: your pharmacist or GP can check the pair against your full medical history.

Read the full methodology
The full report

A 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.

£49 until 30 June 2027 · £79 from 1 July 2027.

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Distil's interactions database is reviewed and updated every quarter. We grade evidence transparently and publish our methodology, including every database change, at /about/methodology. This tool is information, not a substitute for clinical judgement. If you take medication and supplements together, your GP or pharmacist can review your full regimen against your medical history. If you want a full personalised stack reasoned against this same database, the Distil report is the next step up.