Short answer: Calcium, iron, magnesium, multivitamins with minerals and antacids are usually kept about four hours away from levothyroxine because they can bind the tablet in the gut. Vitamin D and B12 alone are less likely to interfere. Biotin does not block the tablet but can distort thyroid blood tests, so many labs advise pausing it beforehand.
Free tool: Levothyroxine timing checker — enter your pill time and see the earliest safe time for coffee, food, calcium and iron.
Medical disclaimer. This page is educational and is not medical advice. Do not start, stop or re-time any prescription or supplement without speaking to your prescriber or pharmacist. Seek prompt medical care if you notice a racing or irregular heartbeat, chest pain, severe fatigue, confusion, or rapid unexplained weight change while on thyroid hormone — these can signal a dose problem.
This table summarises the most common supplements people ask about. Spacing intervals come from product labelling, the American Thyroid Association treatment guideline and the studies cited below. Your pharmacist can confirm them for your exact products.
| Supplement | Interferes with tablet absorption? | Usual spacing from levothyroxine | Notes |
|---|---|---|---|
| Calcium (carbonate or citrate) | Yes — documented in trials | About 4 hours | Includes calcium-fortified juice and chewable antacids |
| Iron (ferrous sulfate and others) | Yes — documented in trials | About 4 hours | Common in prenatal vitamins |
| Magnesium | Likely — binds in the gut | About 4 hours | Includes magnesium-based antacids and laxatives |
| Multivitamin with minerals | Likely — contains calcium, iron, zinc | About 4 hours | The mineral content is the issue, not the vitamins |
| Zinc | Possible | About 4 hours | Often bundled into "thyroid support" blends |
| Vitamin D (alone) | Not shown to block absorption | No strict rule; many take it with lunch | Check the label — some vitamin D products include calcium |
| Vitamin B12 (alone) | Not shown to block absorption | No strict rule | Low B12 is common alongside autoimmune thyroid disease |
| Vitamin C | Not shown to block; small studies suggest it may help absorption in some people | No strict rule | Not a reason to change your routine without your doctor |
| Biotin (B7) | Does not block absorption | Pause before blood tests, as your lab advises | Can make TSH and T4 results look falsely high or low |
| Iodine or kelp | Not an absorption issue | Discuss before use | High iodine intake can aggravate autoimmune thyroiditis |
Use the free Levothyroxine Timing Checker to check your own morning routine against these intervals.
Levothyroxine is absorbed mainly in the small intestine, and the amount absorbed is sensitive to what else is in the gut at the same time. Positively charged minerals — calcium, iron, magnesium, aluminium — can bind the hormone and carry it out unabsorbed.
The two best-documented examples are calcium and iron:
The American Thyroid Association treatment guideline lists calcium, iron and several other agents among those that can reduce levothyroxine absorption and recommends separating them from the dose (Jonklaas et al., Thyroid, 2014).
Biotin is sold for hair, skin and nails, often at doses many times the daily requirement. It does not affect how your tablet works. It can, however, interfere with the laboratory method many hospitals use to measure TSH, free T4 and thyroid antibodies.
A study of healthy adults taking high-dose biotin found clinically meaningful errors across several hormone tests, including thyroid tests (Li et al., JAMA, 2017). The US Food and Drug Administration has issued a safety communication warning that biotin can cause falsely high or falsely low lab results.
Why it matters: a distorted result can lead to a dose change you did not need. Tell your clinician about any biotin — including in hair-skin-nail blends and B-complex products — and ask how long to pause it before your next blood draw. The NIH Office of Dietary Supplements biotin fact sheet covers this interference in more detail.
Questions such as "can I take vitamin D and levothyroxine together" and "can I take B12 with levothyroxine" are among the most searched on this topic. Neither vitamin, taken alone, has been shown to bind levothyroxine in the way calcium or iron does.
Two cautions still apply:
For the evidence behind individual supplements, see the Hashimoto's supplements guide and the detailed page on selenium for Hashimoto's.
These are examples to discuss with your prescriber, not prescriptions.
| Routine | Levothyroxine | Breakfast and coffee | Minerals (calcium, iron, magnesium, multivitamin) |
|---|---|---|---|
| Morning dose | On waking, with water | 30–60 minutes later | With lunch or dinner |
| Bedtime dose | At bedtime, about 3 hours after dinner | Normal morning | With breakfast or lunch |
| Night-shift worker | On waking from main sleep | 30–60 minutes later | With the main meal, 4+ hours from the dose |
Bedtime dosing was associated with thyroid levels at least as good as morning dosing in a randomized trial (Bolk et al., Archives of Internal Medicine, 2010). Switching should still be agreed with your doctor, because your labs may need rechecking after any change. The coffee and food side of the same timing question is covered in levothyroxine and coffee.
The free printable Food Safety & Timing Guide on this page puts the spacing chart on one sheet for your fridge.
If TSH stays out of range despite consistent timing, other causes are worth raising with your doctor: coeliac disease, low stomach acid, proton-pump inhibitors, pregnancy, weight change or a recent switch in tablet brand. Supplements are a common cause of unstable results, but not the only one.
People with Hashimoto's who also want help with day-to-day eating can start with the Hashimoto's breakfast ideas built around the medication window, or see the full Hashimoto's hub. If digestive symptoms are part of the picture, the SIBO diet guide explains a related cause of malabsorption.
Vitamin D on its own has not been shown to block levothyroxine absorption. Many products combine vitamin D with calcium, though, and calcium is usually kept about four hours away. Check the label and confirm with your pharmacist.
B12 alone is not known to interfere with absorption. If your B12 is part of a multivitamin with minerals, the minerals are the reason to space it. Ask your clinician to test B12 before supplementing.
Magnesium, like calcium and iron, can bind levothyroxine in the gut and reduce how much is absorbed. A gap of about four hours is the common advice. Magnesium-based antacids and laxatives count too.
Biotin can make TSH and T4 results appear falsely high or low with some laboratory methods. Many labs advise pausing it for a period before testing. Ask your clinician or lab how long to stop for your dose.
No common vitamin is generally off-limits. The usual cautions are spacing minerals from your tablet, telling your clinician about biotin before tests, and avoiding high-dose iodine or kelp unless prescribed.
Sources: Singh et al., JAMA (2000); Campbell et al., Annals of Internal Medicine (1992); Jonklaas et al., American Thyroid Association guidelines, Thyroid (2014); Li et al., JAMA (2017); Bolk et al., Archives of Internal Medicine (2010); US FDA safety communication on biotin and lab tests (2019); NIH Office of Dietary Supplements fact sheets on biotin and vitamin B12. Compiled by the Nutrition.io Editorial Team from the peer-reviewed sources cited above.
Free printable: Food Safety & Timing Guide (PDF) — thyroid and gut food evidence, the levothyroxine and supplement timing chart, a shopping checklist and a one-week symptom log.