Medication · thyroid hormone

Supplements and Levothyroxine.

Every documented pair, every citation. Below: 4 documented pairs grouped by mechanism.

Levothyroxine, sold under the brand names Eltroxin, Synthroid, Levoxyl, is thyroid hormone replacement. Absorption is reduced by polyvalent cations and high-fibre supplements.

Below are the 4 documented pairs we have explicitly assessed against Levothyroxine in the Distil database: 4 amber. The pairs cluster around 1 mechanism: Mineral chelation (absorption). Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not on this list is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by supplement, and the missing-item form at the bottom of the page routes any uncatalogued supplement into our next curation pass.

Documented interactions

Mineral chelation (absorption)

Amber Calcium

Calcium can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually solves it.

PMID 10838651 · BNF: Levothyroxine

Amber Iron

Iron can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually solves it.

BNF: Levothyroxine

Amber Magnesium Glycinate

Magnesium can bind levothyroxine in the gut and reduce how much you absorb, in the same way that calcium and iron do. Take levothyroxine first thing on an empty stomach and any magnesium supplement at least four hours later (or at bedtime).

BNF: Levothyroxine

Amber Zinc Bisglycinate

Zinc can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually avoids it.

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 (food-supplement interactions only; for drug-drug interactions, the BNF is authoritative). Use the checker below to surface any supplement, and submit a missing item if you take something we have not catalogued.

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

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How we decide

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 methodology

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If a supplement or medication you take isn't in our autocomplete, tell us and we'll add it in the next quarterly update.

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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 comprehensive personalised stack reasoned against this same database, the Distil report is the next step up.