Supplements and Carbimazole.
Carbimazole, sold under the brand name Neo-Mercazole, is an antithyroid agent used in hyperthyroidism.
Carbimazole 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 4 documented pairs we have explicitly assessed against Carbimazole in the Distil database: 4 amber. The pairs cluster around 2 mechanisms: Antithyroid drug interference and Additive thyroid-hormone effect. 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
Antithyroid drug interference
Carbimazole works by lowering an overactive thyroid. Bacopa may gently raise thyroid hormone, so in theory it could pull in the opposite direction and make carbimazole 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 carbimazole, mention bacopa to the clinician who manages your thyroid and keep to your usual thyroid blood tests so your dose can be reviewed if needed.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Carbimazole 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.
Carbimazole 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 carbimazole and make it a little less effective. This has not been tested in people on carbimazole, so it is a caution rather than a firm warning. If you take carbimazole, mention L-tyrosine to the clinician who manages your thyroid and keep to your usual thyroid blood tests.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Additive thyroid-hormone effect
Carbimazole is given to bring an overactive thyroid down. Ashwagandha tends to push thyroid hormone the other way, up, so it can work against your treatment. It is best not to add ashwagandha while you are being treated for an overactive thyroid. If you already take both, tell your GP and have your thyroid bloods checked.
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.
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 evidence-backed compounds worth adding, and the ones worth dropping
It's a paid report: £79, or £49 for the first 25 customers. The interactions check is one section of it, and you can read a real one in full before you buy.
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