The short answer
Taking iron at the same time as oral levothyroxine can reduce its absorption and worsen thyroid control. Direct clinical evidence is strongest for ferrous sulfate, and other iron preparations are not equally studied.
What this answer covers
Evidence concerns oral levothyroxine tablets, principally with ferrous sulfate; injectable levothyroxine is outside this finding.
- Supplement or preparation
- Iron
- Medication ingredient
- levothyroxine
Evidence and practical considerations
Absorption & effectiveness
A direct prospective patient study, complex-formation mechanism, guideline summaries, and current labeling consistently support clinically relevant loss of oral levothyroxine efficacy with simultaneous ferrous sulfate.
- Exact scope
- iron supplements with levothyroxine
- Medication form and route
- oral levothyroxine tablets
- Timing or duration
- Simultaneous daily dosing produced loss of biochemical control within 12 weeks.
What remains uncertain
- Direct clinical evidence is strongest for ferrous sulfate, not every iron salt.
- Evidence applies to oral levothyroxine; no injectable-route interaction is expected.
- Product-label evidence requires the documented levothyroxine-sodium active-moiety bridge.
Factors that may matter: iron salt; iron dose; same-time dosing; oral formulation; baseline TSH stability.
Read the supporting source summaries
Source 1: peer-reviewed prospective human interaction study
pubmed.ncbi.nlm.nih.gov · Source check Jul 24, 2026
Population: Fourteen patients with hypothyroidism stable on levothyroxine.
Mean TSH increased from 1.6 to 5.4 mIU/L and nine patients developed increased hypothyroid symptoms; an insoluble ferric-thyroxine complex was observed in vitro.
Small, nonrandomized study of one iron salt; no long-term outcomes.
Source 2: current U.S. regulatory product labeling
dailymed.nlm.nih.gov · Source check Jul 24, 2026
Population: Patients using oral levothyroxine-sodium tablets.
Label identifies ferrous sulfate as an absorption-interfering agent and instructs at least four hours of separation.
Product-specific sodium-tablet label; evidence summary does not quantify all iron formulations.
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: a8da44f5dbfb8716
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.