Supplements and Propylthiouracil.
Propylthiouracil is an antithyroid agent used in hyperthyroidism.
Propylthiouracil is an antithyroid agent used in hyperthyroidism, typically Graves disease. UK endocrinology prescribes mostly carbimazole, or in pregnancy and certain other contexts propylthiouracil. Mechanism is interference with thyroid hormone synthesis at the level of thyroid peroxidase. The supplement surface is small because antithyroid drugs have minimal CYP metabolism. But two points sit at the centre. Iodine status matters. High iodine intake from kelp or iodine supplements opposes the antithyroid drug effect and is contraindicated. Selenium has been studied in Graves disease with modest evidence of benefit for orbital signs. The UK endocrinology consensus is that patients deficient in selenium may benefit, but routine selenium at high doses for everyone on antithyroid medicine is not supported. Carbimazole carries a small agranulocytosis signal that warrants prompt review of any unexplained fever or sore throat. Supplement choices do not change this monitoring.
Below are the 3 documented pairs we have explicitly assessed against Propylthiouracil in the Distil database: 3 amber. The pairs cluster around 1 mechanism: Antithyroid drug interference. 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
Last reviewed 17 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.
Antithyroid drug interference
Amber Does Bacopa Monnieri interact with ?
Propylthiouracil works by lowering an overactive thyroid. Bacopa may gently raise thyroid hormone, so in theory it could pull in the opposite direction and make propylthiouracil a little less effective. This is based on animal studies rather than people, so it is a sensible caution rather than a firm warning. If you take propylthiouracil, mention bacopa to the clinician who manages your thyroid and keep to your usual thyroid blood tests.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Iodine interact with ?
Propylthiouracil is used to calm an overactive thyroid. Taking an iodine supplement at the same time can work against it by giving the thyroid more raw material to make hormone, which may make your overactive thyroid harder to control. Iodine supplements are generally not advised while you are being treated for an overactive thyroid unless a specialist has told you otherwise.
Evidence: Supported by human study data.
Amber Does L-Tyrosine interact with ?
Propylthiouracil works by lowering an overactive thyroid. L-tyrosine is a building block of thyroid hormone and at high doses can nudge thyroid activity up, so in theory it could pull against propylthiouracil and make it a little less effective. This has not been tested in people on propylthiouracil, so it is a caution rather than a firm warning. If you take propylthiouracil, mention L-tyrosine to the clinician who manages your thyroid and keep to your usual thyroid blood tests.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
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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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.
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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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