Vitamins With Levothyroxine: Safe vs Block List + 4-Hour Rule

Calcium, iron, and antacids (including those with magnesium or aluminum) lower levothyroxine absorption, and the Synthroid label says not to take levothyroxine within 4 hours of them. Some magnesium supplements may lower it too. Coffee and soy also reduce absorption. Biotin and zinc are not on the label's list of absorption blockers, but biotin can throw off thyroid lab results and high-dose iron can lower zinc absorption. ClinCalc DrugStats ranks levothyroxine the second most prescribed drug in the United States in 2024. If you take a multivitamin with your thyroid pill in the morning, you may be absorbing less of your dose than you think.

This article is for informational purposes only and does not replace medical advice. Consult your doctor or pharmacist for advice specific to your medications.

The 4-hour rule at a glance

Why levothyroxine is so sensitive to supplements

Levothyroxine (brand names include Synthroid, Levoxyl, Tirosint) is a synthetic version of the T4 thyroid hormone. It has a narrow therapeutic window, meaning even small changes in how much your body absorbs can shift your thyroid levels from controlled to symptomatic.

The problem: several common minerals bind to levothyroxine in the gut, forming complexes your body absorbs poorly, so that part of the dose passes through without reaching your bloodstream. These interactions can cut how much levothyroxine you absorb. In one study of eight healthy adults, taking calcium carbonate, calcium citrate or calcium acetate at the same time as levothyroxine reduced absorption by about 20% to 25%.

This is not a theoretical concern. In the calcium and iron studies below, TSH rose after people on a stable dose started taking calcium or iron with their levothyroxine. If your TSH climbs after you start a supplement or switch multivitamins, ask your doctor whether the timing could be the reason. (If you recently missed a dose or took a double, see our guides on missed dose of levothyroxine and accidentally took double dose of levothyroxine.)

Supplements that block levothyroxine (4-hour rule)

These lower how much levothyroxine you absorb. The Synthroid label says not to take levothyroxine within 4 hours of iron and calcium supplements and antacids.

Calcium supplements

In a study in JAMA, 20 people on levothyroxine took 1,200 mg of calcium a day as calcium carbonate with their thyroid pill for 3 months. Their free T4 fell and their average TSH rose from 1.6 to 2.7 mIU/L.

If you take a multivitamin, check the label. Multivitamins commonly contain about 200 to 300 mg of calcium, according to the NIH Office of Dietary Supplements.

Iron supplements

Iron causes the same problem. In a study published in Annals of Internal Medicine, 14 people with hypothyroidism took a 300 mg ferrous sulfate tablet with their levothyroxine every day for 12 weeks. Their average TSH rose from 1.6 to 5.4 mU/L, and nine had more hypothyroid symptoms.

This creates a scheduling headache because both levothyroxine and iron absorb best on an empty stomach. See our guide on iron supplement timing for strategies on fitting both into your day.

Magnesium supplements

Magnesium can lower how much levothyroxine you absorb, but the evidence differs by form. The Synthroid label lists antacids with magnesium hydroxide (like milk of magnesia) among products that may reduce absorption, and it says not to take Synthroid within 4 hours of antacids. In one small study in healthy adults, magnesium aspartate powder dissolved in water lowered absorption by about 12%, and the authors say the result cannot be applied to tablets. Magnesium citrate showed a smaller drop that was not statistically significant, but the authors still advise taking levothyroxine apart from magnesium products. A 1999 case report described a patient on levothyroxine whose TSH rose sharply while taking magnesium oxide for constipation, then fell again after it was stopped. We did not find a study of magnesium glycinate. Ask your pharmacist how far apart to take yours. For more on magnesium timing, see best time to take magnesium.

Aluminum-containing antacids

Not a vitamin, but antacids belong on this list. A 2021 review lists the same mechanism for aluminum hydroxide as for calcium and iron: it binds levothyroxine in the gut. Separate by 4 hours, as the label says for antacids.

Supplements and foods that reduce absorption (1 to 2 hour rule)

These also lower how much levothyroxine you absorb, but the advised gaps are shorter.

Coffee

Coffee reduces levothyroxine absorption by 27 to 36% when consumed at the same time. In that study, published in Thyroid, taking levothyroxine with water and having coffee 60 minutes later gave about the same absorption as water alone.

The fix is simple: take levothyroxine with a full glass of water, then wait at least 60 minutes before your first cup of coffee, as a 2021 review of food interactions advises. See our full guide on medication and coffee timing.

Soy products and supplements

Soy can interfere with levothyroxine absorption and may raise the dose some people need, a 2006 review found. It found little evidence that soy harms thyroid function in healthy adults with enough iodine, and said adults with hypothyroidism need not avoid soy foods. The 2021 review advises separating soy protein and levothyroxine by 1 hour. Its evidence comes from soy protein shakes and soy infant formula.

High-fiber supplements

Fiber supplements like psyllium can trap levothyroxine and reduce absorption, and the Synthroid label lists dietary fiber among foods that may bind it. The Metamucil (psyllium) label says to "Take this product 2 or more hours before or after other drugs." A high-fiber breakfast such as bran cereal can have a similar effect if eaten too close to your thyroid medication.

Biotin and thyroid tests: a separate problem

Biotin (vitamin B7) does not reduce levothyroxine absorption. It interferes with a different thing: your blood tests.

Many TSH, free T4, and free T3 lab assays use a biotin-streptavidin binding system. Supplemental biotin can throw off those assays, producing falsely low TSH and falsely high T4 or T3. Your thyroid may actually be on target, but the lab result suggests you are overdosed. That can lead to unnecessary dose reductions.

The Synthroid label and the American Thyroid Association both say to stop biotin for at least 2 days before thyroid testing. In a study of 10 healthy adults, how long biotin skewed results depended on the dose and the lab's testing machine, so if you take a high-dose hair or nail product (usually 5,000 to 10,000 mcg, the ATA notes), ask your lab whether to stop sooner.

Biotin does not need a 4-hour window from levothyroxine. Just hold it before lab work.

Does zinc interfere with levothyroxine?

Zinc helps regulate the enzymes that convert T4 to T3, a 2019 review explains. It is not on the Synthroid label's list of products that lower levothyroxine absorption, so the 4-hour rule does not apply to it.

Iron can get in zinc's way: taking 25 mg or more of iron at the same time as zinc can lower zinc absorption, the NIH Office of Dietary Supplements says. If you take both, take them at different times, such as iron in the morning on an empty stomach and zinc with dinner.

The timing is easy: levothyroxine first thing in the morning, zinc at lunch, dinner, or bedtime. Our full guide on zinc and levothyroxine covers the evidence in detail.

Is vitamin D safe with levothyroxine?

Vitamin D is not on the list of products that lower levothyroxine absorption in the Synthroid label, and it has no 4-hour rule. In a 12-week trial in 201 people on stable levothyroxine doses, weekly high-dose vitamin D lowered TSH by 0.4 mIU/L on average, while TSH rose 0.1 with placebo. The trial did not test whether taking the two at the same time changes absorption. Check the bottle first: some vitamin D products also contain calcium, and calcium does need the 4-hour gap. The label says to take levothyroxine "on an empty stomach, one-half to one hour before breakfast", so vitamin D fits with breakfast or later in the day.

In a 2015 analysis of 20 studies, people with autoimmune thyroid disease, such as Hashimoto's, were more likely to be low in vitamin D than people without it. Your doctor may recommend supplementation based on a 25-hydroxyvitamin D blood level. See can you take too much vitamin D for upper-limit guidance.

Vitamins that are safe with levothyroxine

Not everything interferes. These supplements are not on the Synthroid label's list of products that lower levothyroxine absorption, so they have no 4-hour rule:

Even though these have no 4-hour rule, always double-check the full list of what you are taking with your pharmacist. Pillo handles the tricky part: a persistent empty-stomach alarm for your levothyroxine and separate timers for supplements that need a 4-hour gap.

The 4-hour rule in practice: sample schedules

Here are two schedules that fit levothyroxine into a day with supplements.

Schedule A: early riser

Schedule B: no morning window

If you cannot take levothyroxine first thing (shift work, early feeding schedule, etc.), a bedtime option works. A 2010 trial in Archives of Internal Medicine found that bedtime dosing lowered TSH by 1.25 mIU/L on average compared with morning dosing. The American Thyroid Association guidelines recommend taking it either 60 minutes before breakfast or at bedtime, 3 or more hours after the evening meal.

See our detailed guide on best time to take levothyroxine for more schedule variations and tips.

The multivitamin trap

You take levothyroxine at 7 AM. You take your multivitamin at 7 AM. Your multivitamin contains calcium (300 mg), iron (18 mg), and magnesium (100 mg). The calcium and iron can bind to levothyroxine in the gut, and magnesium may lower absorption too, so part of your dose passes through without being absorbed.

Your TSH slowly rises over weeks. The problem is not your thyroid. The problem is your multivitamin schedule.

The fix: Move your multivitamin to lunch or dinner, at least 4 hours after levothyroxine. Or switch to individual supplements you can time precisely. Pillo keeps the two alarms separate: one persistent alarm at 6 AM for levothyroxine, another at noon for supplements. Neither gets skipped because the alarm repeats until you confirm.

What the Synthroid label says

The FDA prescribing information for Synthroid (section 17, patient counseling) puts it in two sentences: "Inform patients that agents such as iron and calcium supplements and antacids can decrease the absorption of levothyroxine. Instruct patients not to take SYNTHROID tablets within 4 hours of these agents." A multivitamin often carries both iron and calcium, so the 4-hour instruction covers it through the minerals it names.

Watch for hidden iron in prenatal and "complete" multivitamins

Iron hides in places you might not expect. Multivitamins with iron, especially those designed for women, typically carry 18 mg, the NIH Office of Dietary Supplements says, and prenatal vitamins often contain it too. Senior or 50+ formulas often have little or none. Flip the bottle over and read the supplement facts panel. If iron or calcium is listed, the 4-hour rule applies. If you are pregnant and taking levothyroxine, mention the prenatal to your prescriber. The Synthroid label calls for TSH and free T4 checks as soon as pregnancy is confirmed and at least once each trimester.

Thyroid-supportive supplements (worth knowing)

Some supplements are marketed specifically for "thyroid support." A few notes:

• Selenium: Selenium lowered thyroid antibody levels in some trials in Hashimoto's, but a Cochrane review judged the evidence for or against it incomplete and not reliable enough to guide treatment. Selenium is not on the Synthroid label's list of products that lower levothyroxine absorption. If you take it, check the amount: see is 200 mcg of selenium too much.
• Iodine: The ATA guidelines found little evidence that large iodine doses help people with normal thyroid levels, and note that iodine supplements can cause hyperthyroidism, hypothyroidism or thyroiditis. Ask your doctor before taking iodine or kelp.
• Ashwagandha: In a small 8-week trial in 50 adults with subclinical hypothyroidism, it lowered TSH and raised T3 and T4. Talk to your provider before starting it, especially if you take levothyroxine or have hyperthyroidism.

FAQ

Can I take my multivitamin with levothyroxine?

No, not if your multivitamin contains calcium, iron, or magnesium (most do). Take levothyroxine first thing in the morning on an empty stomach, and move your multivitamin to at least 4 hours later. Lunch is the easiest option for most people.

Are prenatal vitamins a problem with levothyroxine?

Prenatal vitamins deserve extra attention because they often contain iron, and some contain calcium, the two minerals the Synthroid label names as reducing levothyroxine absorption. Keep the prenatal at least 4 hours away from your thyroid dose, and tell your prescriber you take one. Consult your doctor or pharmacist about timing that fits your pregnancy care.

How long after levothyroxine can I take calcium?

Wait at least 4 hours. Calcium binds to levothyroxine in the gut and lowers how much you absorb. The label's 4-hour rule names calcium supplements and antacids like Tums. Calcium-fortified orange juice and dairy also carry calcium, so ask your pharmacist whether to space them the same way. See medication and milk interactions for more detail.

Does vitamin D interfere with thyroid medication?

Not according to the label. Vitamin D is not on the Synthroid label's list of products that lower levothyroxine absorption, so it has no 4-hour rule. Check the bottle first: some vitamin D products also contain calcium, and calcium does need the 4-hour gap. See can you take too much vitamin D for dosing guidance.

How long should I stop biotin before a TSH test?

Stop biotin, including multivitamins that contain it, for at least 2 days before a thyroid blood test (TSH, free T4, free T3), as the Synthroid label says. If you take a high dose, such as a 5,000 to 10,000 mcg hair or nail product, ask your lab whether to stop sooner. Biotin does not interfere with levothyroxine itself, only with the lab assay.

Does zinc interfere with thyroid medication?

Not the way iron and calcium do. Zinc is not on the Synthroid label's list of absorption blockers, so the 4-hour rule does not apply, but spacing them by a few hours is a sensible precaution. Iron can lower zinc absorption when the two are taken together, so if you take both supplements, take them at different times.

Can I take magnesium glycinate with levothyroxine?

We did not find a study of magnesium glycinate with levothyroxine. In one small study, another form, magnesium aspartate powder dissolved in water, lowered levothyroxine absorption by about 12%, and the authors advise taking levothyroxine apart from magnesium products. The Synthroid label says not to take Synthroid within 4 hours of antacids, which include magnesium hydroxide. Ask your pharmacist how far apart to take your magnesium. Taking magnesium at bedtime and levothyroxine first thing in the morning puts many hours between them.

What if I accidentally took my supplements with levothyroxine?

Do not take a second dose of levothyroxine. One day of reduced absorption is unlikely to cause problems, because levothyroxine has a long half-life of about 7 days. Resume your correct timing schedule the next day. If this has been happening regularly over weeks or months, tell your doctor. Your TSH may need to be rechecked and your levothyroxine dose may need reviewing.

Is Tirosint less affected by food and supplements?

Tirosint is a capsule filled with liquid levothyroxine. In a small study published in Endocrine, 8 patients whose tablets were affected by coffee switched to a soft gel capsule, and their TSH no longer differed whether they had coffee within 5 minutes or an hour later. However, the Tirosint label still says not to take it within 4 hours of iron, calcium supplements and antacids.

Related guides:

• Best time to take levothyroxine
• Missed dose of levothyroxine
• Accidentally took double dose of levothyroxine
• Iron supplement timing (and what to avoid taking with it)
• Supplements you should not take together
• What vitamins to take together
• Medication and coffee: how long to wait
• Medication and milk interactions
• Best time to take magnesium

If remembering doses is the hard part, a pill reminder app that keeps alarming until you respond can take that job off your plate.

This article is for informational purposes only. It is not medical advice.

Reviewed under our Medical Review Policy.

For the biggest mineral interactions in detail, see our dedicated guides on iron and levothyroxine, calcium and levothyroxine, and magnesium and levothyroxine.

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7. Peer-reviewed study (DOI: 10.1001/jama.283.21.2822). study in JAMA
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