Levothyroxine and Supplements: What to Space Out

Take levothyroxine on an empty stomach, and keep calcium and iron supplements at least 4 hours away from it^{1, 2}. In a trial of 8 adults, calcium carbonate, citrate and acetate each cut levothyroxine absorption by about 20% to 25%³. Biotin can distort thyroid blood tests, so stop it at least 2 days before one¹.

Below: what each supplement does, what to do about it, and a daily schedule. Levothyroxine treats hypothyroidism⁴ and is the generic name of brands such as Synthroid. For the conditions it treats, see hypothyroidism and Hashimoto's thyroiditis.

Calcium and iron have the strongest evidence and a label instruction behind them^{2, 5, 4}. Quick reference:

• Calcium (carbonate, citrate, acetate): take at least 4 hours apart^{2, 5}.
• Iron (ferrous sulfate): take at least 4 hours apart^{2, 4}.
• Magnesium: absorption fell 12% with aspartate and 7% (not significant) with citrate in a crossover trial of 15 adults, and the authors advise separating it⁶.
• Chromium picolinate: reduced absorption in 7 volunteers, so separate it by several hours⁷.
• Biotin: it can skew TSH, T4 and T3 results; stop it at least 2 days before testing^{1, 8}.
• Vitamin C: may improve absorption in people with gastritis⁹.
• Vitamin D, zinc and vitamin B12: we found no absorption study, and the abstracts of two systematic reviews do not name them^{10, 11}.
• Ashwagandha and kelp: not absorption problems, but both change thyroid measures themselves^{12, 13}.

If a multivitamin, prenatal vitamin or mineral product lists calcium, iron or magnesium on its label, treat it like those minerals. Our guide on when to take vitamins covers general timing, and multivitamin has the evidence page.

No. Taken together, calcium lowers levothyroxine absorption, and the label asks for a gap of at least 4 hours^{1, 5}. This applies to calcium supplements and to antacids, which the label names alongside iron and calcium supplements¹.

What the studies found

Four studies put numbers on the effect:

• 20 patients, 3 months: 1,200 mg a day of elemental calcium as calcium carbonate, taken with levothyroxine, raised mean TSH from 1.6 to 2.7 mIU/L. In 20% of patients TSH went above the normal range, with a peak of 7.8 mIU/L¹⁴.
• 7 volunteers, one dose: 2.0 g of calcium as calcium carbonate cut absorption of a 1,000 mcg levothyroxine dose from 83.7% to 57.9%¹⁵.
• 8 healthy adults: 500 mg of elemental calcium as carbonate, citrate or acetate each reduced absorption by about 20% to 25%³.
• 10,999 people on levothyroxine in Tayside: in this observational study, TSH rose by 0.27 mU/L on average after calcium was started, and 4.4% had a rise of more than 5 mU/L¹⁶.

Responses vary. In the 8-adult study, one volunteer's absorption did not fall with any form of calcium, while others saw the T4 response drop by as much as 66%³.

Does the form of calcium matter?

Not for this interaction. Citrate and acetate lowered absorption about as much as carbonate³. Milk counts too: 12 oz (355 mL) of 2% milk, which holds about 450 mg of calcium, lowered levothyroxine absorption in 10 healthy adults (p = 0.02 for total exposure)¹⁷. Our guide to calcium-rich foods lists the amounts.

How to space them

Take levothyroxine first thing in the morning and calcium with lunch or dinner. Bedtime levothyroxine is an option: in a trial of 90 analysed patients it lowered TSH by 1.25 mIU/L compared with morning dosing¹⁸, and a meta-analysis of 10 studies found no difference in TSH between the two timings¹⁹. If you switch to bedtime, move the calcium to the other end of the day. Taking calcium and magnesium together as well? See calcium and magnesium supplements.

Yes. Iron probably binds thyroxine in the gut, and the label asks for the same 4-hour gap as calcium^{20, 2, 4}.

Two studies show the size of the effect:

• 14 patients, 12 weeks, no control group: a 300 mg ferrous sulfate tablet taken with the usual levothyroxine dose raised mean TSH from 1.6 to 5.4 mU/L, and 9 patients had more hypothyroid symptoms²⁰.
• 10,999 people in Tayside: in this observational study, TSH rose by 0.22 mU/L on average after iron was started, and 7.5% had a rise of more than 5 mU/L¹⁶.

If you need iron for iron deficiency, do not stop it. Move the iron, not the levothyroxine, and ask for a TSH recheck, as the large Tayside study advises¹⁶.

Yes, with a gap. Magnesium lowered levothyroxine absorption in the only randomized crossover trial we found, but by less than calcium or iron⁶.

The ThyroMag trial gave 15 healthy adults 1 mg of levothyroxine alone or with a magnesium salt⁶:

• Magnesium aspartate: thyroxine exposure fell by 12% (p = 0.002)⁶.
• Magnesium citrate: exposure fell by 7%, which was not statistically significant (p = 0.076)⁶.

The authors suggested the formulation they used may explain why the effect was smaller than for other minerals⁶. No study we found tested glycinate, oxide or threonate against levothyroxine, so no form can be called safer. The authors advise that people with hypothyroidism take levothyroxine separated from magnesium-containing products, especially if they need TSH in a narrow range⁶. Space whichever you take, as you would calcium. Our guide to what not to take with magnesium and the glycinate vs citrate comparison cover the forms, and magnesium has the evidence page.

Biotin is not known to affect levothyroxine absorption, but it can make thyroid blood tests wrong. Many thyroid assays use a biotin-based method, so extra biotin in your blood can push results up or down^{21, 8}.

Four findings set the practical rule:

• In 10 healthy adults taking 5 or 10 mg a day for 7 days, interference lasted 8 hours after 5 mg a day and 1 to 2 days after 10 mg a day on two of three lab platforms. The third showed none⁸.
• A single 10 mg dose has interfered with thyroid tests taken within 24 hours²¹.
• The levothyroxine label tells patients to stop biotin and biotin-containing supplements at least 2 days before TSH or T4 testing¹.
• Falsely low TSH is the typical error, and it can be misread as thyrotoxicosis²¹.

If you take a hair, skin and nails product or a high-dose biotin supplement, stop it at least 2 days before a thyroid blood test and tell whoever orders your TSH¹. The FDA advises providers to ask patients about biotin when results do not match symptoms²¹. Do not change your levothyroxine dose because of a result that does not fit how you feel.

Wait 30 minutes to 1 hour before breakfast, and take the tablet with a full glass of water^{1, 2}. A review lists food, dietary fiber and espresso coffee among the things that interfere²². The research on common breakfast items:

• Breakfast itself: in a crossover trial of 42 completers, TSH averaged 2.89 mIU/L with levothyroxine taken at breakfast versus 1.9 mIU/L when fasting²³.
• Coffee: compared with water, coffee lowered the rise in blood T4 by 36% in thyroid patients and 29% in volunteers²⁴.
• Milk and yogurt: 12 oz of milk lowered absorption in 10 adults¹⁷. We found no study of yogurt, and no study that sets a wait for dairy. Treat both as calcium sources and keep them away from the tablet; the 4-hour rule is for calcium supplements.
• Fiber, soy and walnuts: the label says soybean flour, cottonseed meal, walnuts and dietary fiber may bind levothyroxine¹. In 8 volunteers, absorption averaged 80% with psyllium versus 89% alone, and the authors judged that neither fiber product was likely to cause detectable malabsorption²⁵. Soy evidence is mostly case reports, and a narrative review judged any clinical impact small²⁶. Do not mix crushed tablets with food or soybean infant formula².

Dairy is not harmful to the thyroid. The issue is calcium binding the tablet¹⁷, so the fix is timing.

How long after eating can I take levothyroxine at night?

The label and MedlinePlus give directions for the morning^{1, 2}. The main bedtime trial had patients dose at bedtime¹⁸, but the sources we read give no set wait after dinner. Ask your prescriber for a gap that suits your evening meal.

No vitamin replaces levothyroxine, and the evidence supports testing for a gap before supplementing. Here is what each common one does around levothyroxine.

Vitamin B12

A meta-analysis of 64 studies (28,597 people) found that people with hypothyroidism had lower B12 levels than healthy people, by 60.67 pg/mL on average, and 27% had B12 deficiency²⁷. We found no absorption study for B12. Ask for a B12 test if you are tired despite a normal TSH. See vitamin B12 and B12 deficiency.

Vitamin D

We found no study testing vitamin D against levothyroxine absorption, and two systematic reviews do not name it in their abstracts^{10, 11}. Its thyroid evidence is on our vitamin D for Hashimoto's page.

Selenium

Selenium is a question of benefit, not absorption. A 2024 meta-analysis of 35 studies found it lowered TSH in people with Hashimoto's who were not on thyroid hormone (7 cohorts, 869 people) and lowered thyroid peroxidase antibodies (29 cohorts, 2,358 people), with adverse events comparable to control²⁸. A 2021 review of 23 trials found lower antibodies in levothyroxine-treated adults at 3, 6 and 12 months but no TSH change. It also found a significantly higher risk of reported adverse effects, and concluded the evidence does not justify using selenium for autoimmune thyroiditis²⁹. See selenium for Hashimoto's; the study is summarised in our selenium meta-analysis summary.

Vitamin C

In 31 people with hypothyroidism and gut disease, 500 mg of vitamin C taken with the levothyroxine dose lowered median TSH from 11.1 to 4.2 mIU/L, and TSH normalized in 17 of them (54.8%)⁹. It was a small sequential trial, and a systematic review says larger studies are needed¹⁰. If you already take vitamin C, keep the habit steady and mention it at TSH checks.

Zinc

We found no study testing zinc against levothyroxine absorption, so there is nothing to report.

They do not bind the tablet the way calcium does. The concern is different: some affect thyroid hormone levels directly.

• Ashwagandha: in 50 adults with subclinical hypothyroidism, 600 mg a day of root extract for 8 weeks changed TSH, T3 and T4 compared with placebo¹². On top of levothyroxine, a supplement that moves thyroid hormone levels could in theory push the dose too high. We found no trial of the combination. See ashwagandha and ashwagandha for thyroid disorders.
• Kelp: in 36 healthy adults, kelp capsules for 4 weeks raised TSH in both the low-dose and high-dose groups, and urinary iodine rose with dose¹³. Kelp is a source of iodine; see kelp and iodine.
• Berberine: we found no human study of berberine with levothyroxine, so treat it as unstudied.

Tell your prescriber before starting any of these. A TSH check a few weeks later shows whether your dose still fits¹⁶.

The gap matters most for these groups:

• Anyone taking calcium or iron with levothyroxine now: in Tayside, more than 5 mU/L rises in TSH occurred in 4.4% of calcium users and 7.5% of iron users¹⁶.
• People with thyroid cancer, heart disease or depression: the authors of the calcium-formulation study said a 20% to 25% absorption drop could be clinically important for them³.
• People with gastritis or other gut disease: changes in stomach acidity make levothyroxine harder to control⁹. See malabsorption.
• People with kidney disease: calcium acetate, a phosphate binder, lowered absorption about as much as calcium carbonate in the 8 healthy adults studied³. MedlinePlus says to take sevelamer, cholestyramine, colesevelam, colestipol or sodium polystyrene sulfonate at least 4 hours after levothyroxine².
• Pregnancy: the label says TSH should be monitored and the dose adjusted during pregnancy¹. If a prenatal vitamin lists iron or calcium, keep it 4 hours from levothyroxine².

Report a headache that persists. MedlinePlus lists headache among the side effects, and signs of too much medication include chest pain, a rapid or irregular heartbeat, nervousness and trouble sleeping². Your prescriber orders lab tests to check your response².

Build the day around one fixed levothyroxine time and a 4-hour gap to calcium and iron². An example:

• 6:30 am: levothyroxine with a full glass of water, nothing else².
• 7:00 to 7:30 am: breakfast and coffee, 30 minutes to 1 hour later¹. Save milk and yogurt for later in the day if you can.
• 12:30 pm or later: calcium, iron and magnesium with lunch, at least 4 hours after the levothyroxine².
• Bedtime option: if you dose at bedtime, put calcium, iron and magnesium at least 4 hours before it instead^{2, 18}.

Keep the same times every day, because steady habits keep TSH steady. Recheck TSH after you start or stop a supplement on this list.

Supplements in this guide

• Calcium
• Iron
• Magnesium
• Biotin (Vitamin B7)
• Selenium
• Vitamin C

Related conditions

• Hypothyroidism
• Hashimoto's Thyroiditis

We searched the research literature for levothyroxine with calcium, iron, magnesium, biotin, vitamin C, vitamin D, selenium, B12, kelp, ashwagandha, berberine, coffee and dosing time. We read the PubMed records of 24 journal articles, the FDA-approved levothyroxine label on DailyMed, NIH Office of Dietary Supplements fact sheets for calcium, iron and biotin, and the MedlinePlus drug page. We prioritised systematic reviews, randomized trials and pharmacokinetic studies in humans.

Most absorption studies are small and use single doses, and several results come from case reports and observational data. We found no human data for berberine, zinc, vitamin D or vitamin B12 against levothyroxine absorption. This guide does not replace advice from your prescriber. Last researched 29 October 2026.

1. DailyMed (U.S. National Library of Medicine). Levothyroxine sodium tablets: FDA prescribing information. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1e11ad30-1041-4520-10b0-8f9d30d30fcc
2. MedlinePlus (U.S. National Library of Medicine). Levothyroxine: drug information. Last revised 15 February 2019. https://medlineplus.gov/druginfo/meds/a682461.html
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4. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
5. National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/
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27. Benites-Zapata VA, et al. Vitamin B12 levels in thyroid disorders: A systematic review and meta-analysis. Front Endocrinol (Lausanne). 2023;14:1070592. doi:10.3389/fendo.2023.1070592
28. Huwiler VV, et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024;34(3):295-313. doi:10.1089/thy.2023.0556 Our study summary.
29. Qiu Y, et al. Insufficient evidence to support the clinical efficacy of selenium supplementation for patients with chronic autoimmune thyroiditis. Endocrine. 2021;73(2):384-397. doi:10.1007/s12020-021-02642-z