Food and medicine interaction
Calcium carbonate and iron supplements and levothyroxine (Synthroid, Levoxyl, others)
EstablishedEvidence level: established. Shown in drug labeling or in controlled studies in people.
Last reviewed
What can happen
Calcium carbonate (including Tums) and iron supplements can reduce how much levothyroxine is absorbed, so thyroid levels may drift low even though you take every dose.
- Why it happens
- Calcium carbonate and iron interfere with absorption of levothyroxine from the gut, so less of each dose gets into the body.
- At ordinary food amounts
- The labeled warning is about supplements and antacids. Calcium-rich foods are not part of that 4-hour rule, though eating anything close to the dose can affect absorption.
What to do
MedlinePlus says to take calcium carbonate or ferrous sulfate at least 4 hours before or 4 hours after levothyroxine. Tell your prescriber if you start or stop these supplements, since your thyroid levels may need rechecking.
Never stop or change a prescribed medicine without talking to the prescriber.
Common questions
Can I have calcium carbonate and iron supplements while taking thyroid hormone?
MedlinePlus says to take calcium carbonate or ferrous sulfate at least 4 hours before or 4 hours after levothyroxine. Tell your prescriber if you start or stop these supplements, since your thyroid levels may need rechecking. The labeled warning is about supplements and antacids. Calcium-rich foods are not part of that 4-hour rule, though eating anything close to the dose can affect absorption.