Supplements and Levothyroxine.
Levothyroxine, sold under the brand names Eltroxin, Synthroid, Levoxyl, is thyroid hormone replacement. Absorption is reduced by polyvalent cations and high-fibre supplements.
Levothyroxine is synthetic T4 hormone replacement for hypothyroidism. Around two million people in England take it. Absorption is the single most important variable, and it is fragile. The drug must be taken on an empty stomach (usually 30 to 60 minutes before breakfast, or at bedtime four hours after the last meal) because food drops absorption by 30 to 40 percent. Polyvalent cations (calcium, iron, magnesium) chelate levothyroxine directly in the gut and need a strict four hour separation. This is the most common supplement interaction question we see. Soya, soluble fibre supplements, and PPIs all reduce absorption through different routes. The BNF and NICE NG145 both flag this. The clinical signal is a TSH that drifts up while the dose was previously stable, often six to twelve weeks after starting a new supplement or changing breakfast timing. Annual TSH monitoring is the safety net. Supplement timing is what keeps the dose working between blood draws.
Below are the 22 documented pairs we have explicitly assessed against Levothyroxine in the Distil database: 19 amber and 3 green. The pairs cluster around 9 mechanisms: Absorption interference, Blood-test (assay) interference, Brassica goitrogen effect (theoretical), Mineral chelation (absorption), Additive thyroid-hormone effect, Reduced thyroid-hormone effect, Thyroid status destabilisation, Beneficial combination, and Thyroid cofactor (beneficial). 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.
Absorption interference
Amber Does Chromium Picolinate interact with ?
Chromium picolinate can bind levothyroxine in the gut and reduce how much of your thyroid hormone you absorb if you take them at the same time. This may make your thyroid treatment work less well. Take levothyroxine first thing on an empty stomach and take any chromium supplement several hours later, for example at lunchtime or bedtime.
Evidence: Supported by human study data.
Green Does Psyllium Husk interact with ?
Psyllium taken at the same time as levothyroxine causes only a small reduction in how much levothyroxine you absorb, and a direct study found the difference was not large enough to matter. Levothyroxine is best taken on an empty stomach anyway, so keeping your psyllium to a different part of the day is sensible rather than essential.
Evidence: Supported by human study data.
Blood-test (assay) interference
Amber Does Biotin (high-dose, >5000mcg) interact with ?
High-dose biotin (the amounts in many hair, skin and nail supplements) does not change how levothyroxine works, but it can make your thyroid blood tests read wrong. It can push free T4 and free T3 falsely high and TSH falsely low, which can look like an overactive thyroid that is not really there, or hide a result your GP needs to see to set your dose. Because people on levothyroxine have their thyroid checked regularly, it is sensible to pause high-dose biotin for two to three days before any thyroid blood test, and to tell whoever takes the blood that you have been taking it.
Evidence: Well established across multiple human studies or interaction references.
Amber Does Vitamin B Complex interact with ?
High-potency B complexes can carry enough biotin (B7) to make thyroid blood tests read wrong, pushing free T4 and free T3 falsely high and TSH falsely low. This does not change how levothyroxine works, but because people on levothyroxine have their thyroid checked regularly, a misread can lead to the wrong dose. Standard B complexes rarely contain enough biotin to matter, but if yours is high-potency, pause it for two to three days before a thyroid blood test and tell whoever takes the blood.
Evidence: Supported by human study data.
Brassica goitrogen effect (theoretical)
Amber Does Sulforaphane (Broccoli Sprout Extract) interact with ?
Brassica vegetables (broccoli, cabbage, kale) contain compounds that in theory could slightly affect thyroid hormone production by competing with iodine uptake. A 2024 systematic review found minimal real-world risk in humans with adequate iodine intake, so this is a cautious flag rather than a strong warning. If you are starting sulforaphane while on levothyroxine, ask your GP to check your TSH at the next routine blood draw and make sure you are getting enough iodine.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Mineral chelation (absorption)
Amber Does Calcium interact with ?
Calcium can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually solves it.
Evidence: Supported by human study data.
Amber Does Iron interact with ?
Iron can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually solves it.
Evidence: Supported by human study data.
Amber Does Magnesium interact with ?
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).
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Magnesium Glycinate interact with ?
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).
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Magnesium L-Threonate interact with ?
Magnesium can bind levothyroxine in the gut and reduce how much you absorb, the same way calcium and iron do. Magnesium L-threonate is still elemental magnesium, so it carries the same binding effect. Take levothyroxine first thing on an empty stomach and take any magnesium supplement at least four hours later, or at bedtime.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Zinc interact with ?
Zinc can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually avoids it.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Zinc Bisglycinate interact with ?
Zinc can bind to levothyroxine in the gut and reduce how much you absorb. Separating doses by four hours usually avoids it.
Evidence: Limited human data, based on case reports and a known mechanism.
Additive thyroid-hormone effect
Amber Does Ashwagandha interact with ?
Ashwagandha on top of levothyroxine can tip you into having too much thyroid hormone, because ashwagandha can raise your body's own thyroid output on top of your dose. Watch for a racing heart, feeling wired, or unexpected weight loss, and ask your GP to check your thyroid blood tests if you take both. Do not change your levothyroxine dose yourself.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Bacopa Monnieri interact with ?
Bacopa may gently raise your own thyroid hormone, so taken alongside levothyroxine it could in theory add to your total thyroid level and tip you towards having a little too much, with symptoms like a racing heart, feeling wired, or losing weight. This is based on animal studies rather than human ones, so it is a sensible caution rather than a firm warning. If you take both, mention it to the clinician who manages your thyroid and ask for a TSH blood test a few weeks later so your levothyroxine dose can be reviewed if needed.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does L-Tyrosine interact with ?
L-tyrosine is one of the building blocks your body uses to make thyroid hormone, and a high-dose study found it can gently lower TSH and nudge thyroid activity up. On top of levothyroxine that effect could in theory add up and tip you towards having a little too much thyroid hormone, though this has not been tested in people who take levothyroxine. If you take both, mention the L-tyrosine to the clinician who manages your thyroid and ask for a TSH check so your dose can be reviewed if needed.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Reduced thyroid-hormone effect
Amber Does Alpha Lipoic Acid interact with ?
Alpha lipoic acid may slow the body's conversion of thyroid hormone into its active form, so in theory it could blunt some of the benefit you get from levothyroxine and let a few underactive-thyroid symptoms creep back. This is based on animal studies rather than people, and the evidence is not all one way, so it is not a reason to avoid the combination. If you take levothyroxine, tell the clinician who manages your thyroid before starting alpha lipoic acid and ask for a thyroid blood test (TSH) a few weeks later so your dose can be reviewed if needed.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Holy Basil interact with ?
Holy basil (tulsi) has been reported to lower the thyroid hormone thyroxine (T4), though this has only been shown in animals and not in people. If you take levothyroxine, the concern is that holy basil could work against it and let some underactive-thyroid symptoms come back. There is no proof this happens in humans, but because your thyroid dose is finely balanced, tell the clinician who manages your thyroid before starting holy basil and ask for a TSH check a few weeks later so your dose can be reviewed if needed.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does L-Carnitine interact with ?
L-carnitine can blunt the effect of your thyroid hormone, because it slows the hormone getting into your cells. If you take levothyroxine, supplemental L-carnitine may work against it and let some underactive-thyroid symptoms creep back. Tell the clinician who manages your thyroid before starting it, and ask for a TSH check a few weeks later so your dose can be reviewed if needed.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Lemon Balm interact with ?
Lemon balm has been shown in laboratory studies to dampen the thyroid-stimulating signal, so in theory it could work against your thyroid medication and let some underactive-thyroid symptoms creep back. This has only been seen in the test tube, not in people taking levothyroxine, so it is a precaution rather than a proven problem. If you want to take lemon balm regularly, mention it to the clinician who manages your thyroid and ask for a TSH check a few weeks later so your dose can be reviewed if needed.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Thyroid status destabilisation
Amber Does Iodine interact with ?
Taking extra iodine while you are on levothyroxine can shift your thyroid status and may change how much levothyroxine you need. Most people on a stable dose do not need an iodine supplement, and large doses are best avoided unless your GP has advised one. If you do start iodine, ask your GP to recheck your TSH so your dose can be adjusted if needed.
Evidence: Supported by human study data.
Beneficial combination
Green Does Vitamin C interact with ?
Vitamin C and levothyroxine can be a helpful pairing rather than a problem, especially if you have a stomach condition that lowers your stomach acid. Levothyroxine is absorbed better when the stomach is acidic, and taking it with vitamin C improved thyroid blood results in people whose levels had been hard to control. For most people with a normal stomach this makes little difference, and levothyroxine is still best taken on an empty stomach. If your thyroid levels have been difficult to settle, it is worth raising vitamin C with your prescriber rather than changing anything on your own.
Evidence: Supported by human study data.
Thyroid cofactor (beneficial)
Green Does Selenium interact with ?
Selenium and levothyroxine work together rather than against each other. Selenium is a building block the body needs to convert thyroid hormone into its active form, and in people with Hashimoto's it can lower thyroid antibodies. It does not change how much levothyroxine you absorb or how much you need, and good trials found no safety problem combining them. Keep your selenium to around 100 to 200 micrograms a day, since very high doses are harmful in their own right. We treat this pair as safe to combine.
Evidence: Supported by human study data.
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.
Read the full methodologyA 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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