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Supplements and Thyroid Medicine: What to Keep Apart From Levothyroxine, Biotin Before Blood Tests, and Kelp

Take levothyroxine on an empty stomach and keep calcium, iron and antacids at least 4 hours away, as its label says, and ask for a TSH check when you start or stop them. The label also says to stop biotin at least 2 days before thyroid tests; tell the lab too. Skip iodine, kelp and sea moss products unless your clinician prescribes iodine: excess iodine can upset the thyroid (grade B). Never change your dose yourself.

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This guide is for adults who take levothyroxine (thyroid hormone tablets) and want to know which supplements, foods and drinks get in its way. It also covers iodine and kelp for anyone with thyroid disease, including people on antithyroid medicine.

Never change your levothyroxine dose yourself, and don't stop, skip or swap it for a supplement. The dose is set from blood tests, and the label describes a narrow margin between too little and too much. Tell your prescriber or pharmacist about every supplement you start or stop, and ask whether your TSH should be rechecked when you start or stop calcium, iron, a multivitamin, fibre, a protein shake or an acid reducer.

The short version

  • Calcium, iron and antacids: the levothyroxine label says to keep them at least 4 hours away from the tablet. Calcium and iron swallowed together with it pushed TSH (the blood test that rises when thyroid hormone runs low) up in two small studies of patients. Grade C (Singh 2000; Campbell 1992).
  • Biotin before blood tests: it can make some thyroid tests read falsely high or low. The label says to stop it at least 2 days before TSH or T4 tests; the FDA says to tell your doctor and the lab. Grade B (Li 2017; Ylli 2021).
  • Iodine, kelp and sea moss: too much iodine can cause an underactive or an overactive thyroid, and kelp products vary enormously. Grade B for the harm (Farebrother 2019; Aakre 2021).
  • "Thyroid support" products: 9 of 10 tested contained thyroid hormone. Grade C (Kang 2013).
  • Coffee, fibre, soy and acid-reducing medicines: each lowered absorption or raised TSH in small studies. Grade C (Benvenga 2008; Guzman-Prado 2021).
Supplement, food or drinkWhat it doesOn the levothyroxine label?GradeWhat to do
Calcium, iron, antacidsBind the hormone; TSH rose with calcium and iron in small studies of patientsYes: at least 4 hours apartCTake them 4 hours away from the tablet
Biotin (hair, skin and nail products, some multivitamins)Distorts some thyroid blood tests, not the thyroidYes: stop at least 2 days before TSH or T4 testsBTell your doctor and the lab
Iodine, kelp, sea mossExcess iodine can cause an under- or overactive thyroidNo; NIH and thyroid-society warningsB (harm)Avoid unless a clinician prescribes iodine
"Thyroid support" or glandular productsSome contain T3 and T4NoCAvoid; tell your prescriber if you've used one
CoffeeLess absorbed when swallowed togetherNoCTablet with water; coffee later
Fibre, soy protein, grapefruit juiceLower or delay absorptionYes, as foodsCKeep apart; ask about a TSH check if your diet changes
PPIs and other acid reducersLess absorbed when stomach acid is lowYesCDon't stop them; ask about a TSH check
Chromium picolinateLower absorption in one study of 7 volunteersNoDSeparate by several hours

What supplements should you not take with levothyroxine?

None is off limits for everyone, but several need to be kept apart from it, and a few are best avoided. The label says to take it once a day, preferably on an empty stomach, half an hour to an hour before breakfast, and at least 4 hours before or after medicines that interfere with absorption (levothyroxine label). Only 40% to 80% of each dose is absorbed, helped by stomach acid, so anything that binds the hormone or lowers acid can leave you short. A 2021 systematic review of 63 studies found limited evidence on coffee, soy, fibre, calcium, iron and tube feeds, and all of them lowered absorption (Wiesner 2021).

How far apart should you take calcium, iron or magnesium?

At least 4 hours for calcium, iron and antacids, per the label; grade C for the TSH studies, which were small and had no comparison group. Over 3 months of calcium carbonate swallowed alongside the tablet, 20 patients on a steady dose saw average TSH climb from 1.6 to 2.7, one in five ended up above the normal range, and levels settled once the calcium stopped (Singh 2000). Iron (ferrous sulfate) taken with thyroxine for 12 weeks lifted average TSH in 14 patients from 1.6 to 5.4; 9 of them had more symptoms (Campbell 1992). Magnesium lowered absorption less in one small trial in healthy adults (Attinger 2025); details are on can I take iron, calcium or magnesium with levothyroxine?

Multivitamins and prenatal vitamins often hold calcium and iron, so give them the same gap (iron supplements; do you need a multivitamin?).

Does biotin affect thyroid blood tests?

Grade B: yes, on many common test systems, and a wrong result can mislead your doctor about your dose. Biotin doesn't change your thyroid; it upsets the chemistry of tests that rely on it, so readings can come out falsely high or falsely low. When 13 adults took 10 mg daily, one testing system gave shifted TSH, free T4 and total T3 readings, worst 2 hours after a dose (Ylli 2021). In 6 healthy adults on the same amount for a week, 9 of 23 tests built on biotin went wrong, against none of 14 built without it (Li 2017).

  • The levothyroxine label: pause biotin, including multivitamins that contain it, for 2 days or more before a TSH or T4 test.
  • The FDA: biotin turns up in multivitamins, prenatal vitamins and hair, skin and nail products, many at 20 mg and some at 100 mg a pill, against a daily need of 0.03 mg. It also says there isn't enough information to know whether pausing for any number of hours avoids wrong results, so tell your doctor and the lab what you take (FDA 2019).

Biotin can also make troponin, the blood test for a heart attack, read falsely low. With chest pain, call emergency services and tell the team about it. More in does biotin affect thyroid and troponin tests?

Can coffee, fibre, soy or grapefruit affect levothyroxine?

Grade C: yes, in small studies, which is why the tablet goes down with water on an empty stomach.

  • Coffee. Eight women's TSH stayed too high while they took the tablet with coffee or espresso. In absorption tests, coffee with the tablet cut the amount absorbed by 27% to 36%, while coffee 60 minutes later made no clear difference (Benvenga 2008).
  • Fibre and protein shakes. The label lists soybean flour, cottonseed meal, walnuts and dietary fibre as foods that can bind the hormone, and bran interfered even more than coffee in the study above. One woman needed unusually high doses while drinking a soy protein shake straight after her tablet, until she spaced them out (Bell 2001); in another case, TSH rose after a whey protein supplement was added (Kumarathunga 2021).
  • Grapefruit juice. The label says it may delay absorption and lower the amount absorbed. In 10 healthy volunteers the peak fell by 11%, which the authors judged likely to matter little (Lilja 2005).

If a morning gap is hard to keep, ask your prescriber about timing rather than changing it on your own. Pooling 10 studies, taking the tablet at bedtime did no worse for TSH than taking it before breakfast (Pang 2020).

Do acid-reducing medicines or chromium lower levothyroxine?

Acid reducers: grade C. Chromium: grade D, from one small volunteer study. The label warns that proton pump inhibitors (PPIs, such as omeprazole), antacids and sucralfate can lower absorption. In a systematic review of 7 studies of people on levothyroxine, every study found TSH higher once a PPI was added, significantly so in most (Guzman-Prado 2021). Keep taking the PPI and ask whether your TSH should be rechecked; see medicines that lower nutrients.

Swallowed together, chromium picolinate (sold for blood sugar and weight) lowered how much levothyroxine 7 healthy volunteers absorbed in a one-day test; the authors advised spacing them by several hours (John-Kalarickal 2007).

Is sea moss or kelp safe for your thyroid?

Grade B for the harm: excess iodine can cause thyroid problems, and these products are not a thyroid treatment. Your thyroid needs some iodine, but too much can cause goitre (a swollen thyroid), an underactive or overactive thyroid, or thyroid inflammation, and people with Hashimoto's or other autoimmune thyroid disease can react at intakes that are safe for others (NIH ODS; Farebrother 2019).

  • The dose is a lottery. Among 96 kelp and seaweed products on sale in Norway, supplements gave 5 to 5,600 mcg of iodine per daily dose, 54 products would pass the upper limit used in that study in one portion or dose, and labelling was inadequate or inaccurate for several (Aakre 2021).
  • What happened to people. A woman with a nodular goitre became overactive after 4 weeks of a kelp tea (Müssig 2006); a woman with no thyroid disease went overactive, then underactive, on a kelp diet product (Di Matola 2014). Two people with Hashimoto's became severely underactive on a high-iodine product, and a woman on antithyroid medicine flared on another (Hoang 2013).
  • Sea moss. NIH's LactMed says no medicinal value of sea moss has been shown, that it carries iodine and toxic heavy metals such as arsenic, lead, cadmium and mercury from the water it grew in, and that breastfeeding mothers should avoid it unless those levels are known to be safe (LactMed).

The American Thyroid Association advises against iodine or kelp supplements giving more than 500 mcg of iodine a day and says no thyroid benefit is known from daily iodine above the recommended amount (Leung 2015). Upper limit for adults: 1,100 mcg of iodine a day from food and supplements combined, since more can cause goitre and an under- or overactive thyroid; anything above that needs blood tests or a clinician's advice (NIH ODS; check your stack). Our iodine page grades ordinary-dose trials, where thyroid tests barely changed; the grade B here is for the harm from high doses, as in kelp and iodine products. Our Hashimoto's guide covers iodine in that disease.

Do "thyroid support" supplements contain thyroid hormone?

Grade C: in one lab study, most did. Of 10 thyroid health supplements bought off the shelf, 9 contained T3 and 5 contained T4; taken as labelled, 4 supplied 8.57 to 91.6 mcg of T4 daily, some more than people are commonly prescribed (Kang 2013). A thyroid-extract product caused an overactive thyroid in one reported case (Hoang 2013). On top of levothyroxine, hidden hormone adds to the dose your prescriber set.

Is ashwagandha safe with thyroid medicine?

Grade C: it can raise thyroid hormone, so it isn't a neutral add-on. Against placebo, ashwagandha root extract nudged TSH, T3 and T4 toward normal over 8 weeks in a trial of 50 adults whose thyroid was mildly underactive (Sharma 2018). A 73-year-old woman who used ashwagandha on her own to treat her underactive thyroid for 2 years developed a racing heart and a very low TSH (Kamal 2022). See can I take ashwagandha with levothyroxine?

Does anything help tiredness that lingers on levothyroxine?

Grade C, mixed: nothing has good evidence yet. In 60 people still tired on treatment, L-carnitine (990 mg twice a day for 12 weeks) left the main fatigue score unchanged against placebo, though mental fatigue improved (An 2016). A 2024 review of 7 studies (455 people) found no high-quality evidence for any option tested, including ginger, L-carnitine, adding T3 hormone and surgery (Hidalgo 2024). Ask whether your TSH is in range and whether low iron or B12 could explain it; see supplements for energy and fatigue.

What we'd skip

  • Iodine, kelp and sea moss products with thyroid disease, unless a clinician prescribes iodine: excess has caused both under- and overactive thyroids, and doses vary widely (Leung 2015; Aakre 2021).
  • "Thyroid support" and glandular products: many hold real thyroid hormone (Kang 2013).
  • Treating an underactive thyroid with a herb instead of prescribed care: the ashwagandha case above ended with a racing heart (Kamal 2022).

Safety and interactions

  • Too much thyroid hormone shows as a fast or irregular heartbeat, chest pain, breathlessness, tremor, poor sleep, weight loss, diarrhoea or feeling hot; the label lists these as signs of an excess dose. It warns that over-treatment can bring on angina or an irregular rhythm such as atrial fibrillation, especially in older adults and people with heart disease, and can thin bones over time. Hidden hormone or iodine can cause it.
  • Antithyroid medicine (methimazole and others): high-dose iodine adds to its effect and could tip the thyroid into underactivity; potassium iodide with ACE inhibitors or spironolactone can push potassium too high (NIH ODS).
  • Pregnancy: the label says to check TSH as soon as pregnancy is confirmed, and your prescriber may need to adjust your treatment. Tell your prescriber straight away, and ask your midwife or obstetric team which prenatal vitamin to take and when, since most contain iron and calcium. See prenatal supplements.
  • Surgery: the label tells patients to let their doctor or dentist know they take levothyroxine before any operation; list your supplements then too.
  • Several products at once? Check your stack against the upper limits.

If a supplement seems to clash with your thyroid medicine, keep taking the medicine and let your prescriber or pharmacist decide what changes.

When to see a doctor

  • Go now if you have chest pain, fainting, severe breathlessness or a heartbeat that races or skips, and mention any biotin.
  • Book soon for tremor, unexplained weight loss, feeling hot or sleeping badly after starting a new product (too much hormone), or for tiredness, feeling cold and weight gain coming back (too little).
  • Ask for a TSH check when you start or stop calcium, iron, a multivitamin, a fibre or protein supplement or a PPI, and when you're pregnant or planning to be.
  • See a doctor for a new swelling in your neck or trouble swallowing.

FAQ

Can I take vitamin D with levothyroxine? Vitamin D on its own is not on the label's list of things that interfere with levothyroxine. Many vitamin D products also contain calcium, which is, so check the label and keep calcium 4 hours away from the tablet.

How long after levothyroxine can I drink coffee? The label says to take the tablet on an empty stomach, half an hour to an hour before breakfast. In a small study, coffee 60 minutes after the tablet made no clear difference, while coffee with the tablet cut absorption by 27% to 36%.

Can I take sea moss if I have hypothyroidism? We'd skip it. Its iodine content is unknown and can be high, excess iodine can cause an under- or overactive thyroid, and NIH's LactMed says no medicinal value has been shown and it carries heavy metals.

Should I stop biotin before a thyroid test? The levothyroxine label says to stop biotin at least 2 days before TSH or T4 tests. The FDA says there isn't enough information to know whether any pause avoids wrong results, so also tell your doctor and the lab.

Can I take magnesium with levothyroxine? In one small trial in healthy adults, magnesium lowered absorption a little (Attinger 2025). The label names magnesium only in antacids, so ask your pharmacist about spacing.

Sources

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