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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, especially if you take any medication, are pregnant, breastfeeding, or have a serious health condition.
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