Levothyroxine and Ferrous Sulfate: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Levothyroxine
Ferrous Sulfate
No brand names on record
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced efficacy of levothyroxine and an increased risk of hypothyroidism
Interaction Deep Dive
Concurrent administration of ferrous sulfate with levothyroxine has resulted in reduced levothyroxine serum levels and efficacy4 in hypothyroid patients 25. Levothyroxine should be administered at least 4 hours apart from iron or iron-containing preparations 241. Monitor closely during coadministration and adjust levothyroxine dose if necessary 3.
Why it happens (mechanism)
Reduced levothyroxine absorption
Literature reports
1 report — tap to read
a) One study conducted an uncontrolled clinical trial in which 14 hypothyroid patients who were receiving levothyroxine replacement were also administered 300 mg ferrous sulfate tablets simultaneously with levothyroxine. Assessment at 12 weeks revealed significant increases in serum TSH (from 1.6 +/- 0.4 mU/L to 5.4 +/- 2.8 mU/L, p less than 0.01). A significant reduction in free levothyroxine index and total serum levothyroxine was NOT seen. An increase in the clinical hypothyroidism score was seen in 9 of the 11 patients who had an increase in serum TSH. Formation of insoluble ferric-thyroxin complexes is the probable mechanism which causes reduced levothyroxine absorption 5.
Common questions
Can I take Levothyroxine and Ferrous Sulfate together?
Reduced efficacy of levothyroxine and an increased risk of hypothyroidism Always confirm with your pharmacist or prescriber before making any change.
How serious is the Levothyroxine and Ferrous Sulfate interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Levothyroxine or Ferrous Sulfate need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (5)
- Product Information: LEVO-T® oral tablets, levothyroxine sodium oral tablets. Neolpharma Inc, Caguas, PR, 2026. DailyMed
- Product Information: TIROSINT(R)-SOL oral solution, levothyroxine sodium oral solution. IBSA Pharma Inc. (per FDA), Parsippany, NJ, 2023. DailyMed
- Liu H, Lu M, Hu J, et al: Medications and food interfering with the bioavailability of levothyroxine: a systematic review. Ther Clin Risk Manag 2023; 19:503-523. DOI
- Product Information: SYNTHROID(R) oral tablets, levothyroxine sodium oral tablets. AbbVie Inc (per FDA), North Chicago, IL, 2024. DailyMed
- Campbell NR, Hasinoff BB, Stalts H, et al: Ferrous sulfate reduces thyroxine efficacy in patients with hypthyroidism. Ann Intern Med 1992; 117:1010-1013.
Keep reading about Levothyroxine
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