Key takeaways
The 30-second version
- Calcium and iron block levothyroxine. Keep them at least 4 hours apart from your dose (this is on the drug label).
- Take levothyroxine on an empty stomach with water, and wait 30 to 60 minutes for coffee or breakfast.
- Magnesium, antacids, high-fibre meals, soy and acid-reducing drugs (PPIs) can also lower absorption. Space them out or tell your doctor.
- Biotin does not affect absorption, but it can falsely change thyroid lab results.
- If you change supplements, recheck TSH about 6 to 8 weeks later. Never skip or change your dose without your doctor.
Want a version built around your symptoms and labs?
Take the 3-min quiz →Calcium and levothyroxine should be taken at least 4 hours apart. The Synthroid prescribing information (and the Levoxyl label) advises this spacing for calcium carbonate and ferrous sulfate, because both bind levothyroxine in the gut and reduce how much reaches your blood. The same applies to many multivitamins and "bone support" products that contain calcium or iron. Other items, including coffee, soy, high-fibre foods, antacids and proton-pump inhibitors, have a smaller or less certain effect. This guide shows the size of each interaction, a practical schedule, and when to recheck TSH. For the question of replacing the drug itself, see levothyroxine natural alternatives.
Why Absorption Is Fragile
Levothyroxine is absorbed in the small intestine, and absorption depends on stomach acid and on nothing else being around to bind it. Anything that raises stomach pH (antacids, PPIs), binds the molecule (calcium, iron, fibre) or speeds or slows gut transit can lower the amount absorbed. When absorption drops, TSH drifts up, and you may feel hypothyroid symptoms even though your dose has not changed.
The Interaction Table
| Substance | Effect | Suggested spacing | Evidence |
|---|---|---|---|
| Calcium carbonate | Forms an insoluble chelate; reduces absorption. Singh et al., JAMA 2000: mean TSH rose from 1.6 to 2.7 mIU/L in 20 patients during calcium use | At least 4 hours (label) | B |
| Iron (ferrous sulfate) | Forms a complex; Campbell et al., Ann Intern Med 1992: mean TSH rose from 1.6 to 5.4 mU/L in 14 patients on stable levothyroxine taking ferrous sulfate with it | At least 4 hours (label) | B |
| Bile acid sequestrants (a prescription class) | Bind levothyroxine | Levothyroxine at least 4 hours before them, or monitor TSH (label) | Label |
| Antacids with aluminium or magnesium, sucralfate | Label cautions about reduced absorption and advises monitoring | Commonly 4 hours; ask your pharmacist | C to label |
| Magnesium supplements | No dedicated trial found; extrapolated from antacid data | A 4 hour gap is a cautious default | C |
| Proton-pump inhibitors | Raise stomach pH; label advises monitoring | Spacing may not fix this; recheck TSH, as dose changes are sometimes needed | Label |
| Coffee | Benvenga et al., Thyroid 2008: coffee taken with levothyroxine reduced absorption by roughly 30%; no difference when taken 60 minutes later | Wait 30 to 60 minutes | B (small study) |
| Soy, walnuts, cottonseed meal, high-fibre foods | Listed on levothyroxine labelling as foods that may bind or lower absorption | Take levothyroxine first, keep meals consistent | Label |
| Biotin | No absorption effect; skews immunoassay results | Stop before labs, see biotin and thyroid tests | n/a |
The two best-evidenced items, calcium and iron, are also the ones most people take together in multivitamins, so check labels.
Supplements With No Known Binding Problem
- Selenium and vitamin D: neither is listed as binding levothyroxine. See selenium for Hashimoto's and the broader supplements for Hashimoto's.
- Zinc: limited data. Many clinicians space it by a few hours as a precaution.
An "unlisted" interaction does not guarantee safety. If your TSH moves after starting a new product, suspect the product.
A Practical Daily Schedule
- On waking: levothyroxine with a full glass of water, empty stomach, same time daily.
- 30 to 60 minutes later: coffee and breakfast.
- Lunch: selenium, vitamin D, omega-3 (with food).
- At least 4 hours after levothyroxine, e.g. dinner: calcium, iron, magnesium and multivitamins containing them.
Bedtime dosing, several hours after the last meal, is an option some clinicians use for people who cannot manage a morning empty stomach; ask your doctor, and keep it consistent.
Hashimoto's Supplement Timing Guide
On Waking (before eating)
With Breakfast
With Dinner / Evening
What blocks levothyroxine absorption
When to Recheck TSH
Recheck TSH about 6 to 8 weeks after you start, stop or change the timing of a calcium, iron or other interacting product, or after starting a PPI. This follows standard practice for dose adjustment and allows the new steady state to appear. For reference ranges see Hashimoto's lab targets. Also read iron deficiency in Hashimoto's, since treating low iron is worthwhile but needs spacing.
Do not adjust the levothyroxine dose yourself because of a lab number. Report the pattern to your prescriber.
Frequently Asked Questions
How long should I wait between levothyroxine and calcium?
At least 4 hours, per levothyroxine labelling.
Can I take iron and levothyroxine together?
No. Keep them at least 4 hours apart.
Is coffee a problem?
Coffee taken with the tablet lowered absorption in a small study, and waiting 60 minutes avoided the effect. 30 to 60 minutes is the usual advice.
Take the free 3-minute AutoimmuneFinder quiz for a personalized, evidence-graded protocol that accounts for your medication.
This site provides educational information only. Not medical advice. Always consult your healthcare provider before changing your treatment plan.