The short answer
Guidance recommends taking ferrous sulfate at least four hours before or after oral levothyroxine because it can interfere with absorption. Four hours is the labeled convention; studies have not established the minimum effective interval for every product.
What this answer covers
Applies to iron (II) sulfate, also called ferrous sulfate, with oral levothyroxine tablets or capsules.
- Supplement or preparation
- Ferrous sulfate
- Medication ingredient
- levothyroxine
Evidence and practical considerations
Timing & monitoring
Direct loss-of-effect evidence and current official instructions consistently support separation, with four hours as the prevailing label-based interval.
- Exact scope
- ferrous sulfate or iron supplements with levothyroxine
- Medication form and route
- oral levothyroxine tablets and capsules
- Timing or duration
- Apply whenever ferrous sulfate or a relevant iron product is used on the same day.
What remains uncertain
- The exact four-hour interval is traditional and labeled but not validated against shorter intervals.
- Direct evidence is strongest for ferrous sulfate.
- The instruction is limited to oral levothyroxine.
Factors that may matter: iron salt; same-time dosing; oral formulation; iron dose; baseline TSH control.
Read the supporting source summaries
Source 1: current U.S. regulatory product labeling
dailymed.nlm.nih.gov · Source check Jul 24, 2026
Population: Patients using oral levothyroxine-sodium tablets.
Lists ferrous sulfate as an absorption-interfering binding agent and instructs at least four hours of separation.
Product-specific label; other iron salts are not individually quantified.
Source 2: U.S. government patient medication guidance
medlineplus.gov · Source check Jul 24, 2026
Population: Patients taking levothyroxine.
Directs ferrous sulfate at least four hours before or after levothyroxine.
Does not resolve every iron preparation.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Research summary published |
Article revision: e1d2bb58ead4f914
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.