Class landing · Levothyroxine

Supplements and levothyroxine.

The supplements that chelate levothyroxine and the four-hour separation rule that solves it.

Levothyroxine absorption is reduced by polyvalent cations: calcium, iron, magnesium and zinc all bind it in the gut and reduce how much reaches the bloodstream. This is a chelation interaction, not a toxicity one, and the management is operational: separate the doses by four hours.

Take levothyroxine first thing on an empty stomach. Take any mineral supplement (including a multivitamin containing them) at least four hours later. The same applies to high-fibre supplements like psyllium. Biotin does not affect levothyroxine itself but interferes with thyroid function tests at doses above 5,000 mcg, so flag it before any blood draw.

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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.

Your supplements

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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.

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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.