Levothyroxine and Supplements: What to Know

Levothyroxine is a thyroid hormone replacement with a narrow therapeutic window -- too little or too much absorption meaningfully changes how it works. Its own FDA labeling documents several supplement and medication interactions worth knowing.

Timing-sensitive absorption

Levothyroxine absorption is reduced when taken too close to certain minerals, including calcium and iron. This is a documented, sourced finding, not a general caution -- see Timing & Absorption for the specific separation windows currently on record for levothyroxine.

Documented interaction findings

• Levothyroxine + omeprazole (monitor) -- levothyroxine's own label states that proton pump inhibitors "may cause hypochlorhydria, affect intragastric pH, and reduce levothyroxine absorption," recommending monitoring.
• Levothyroxine + sertraline (monitor, high evidence) -- levothyroxine's label states that sertraline in patients stabilized on levothyroxine "may result in increased levothyroxine sodium capsules requirements."

What this means in practice

None of this means levothyroxine cannot be combined with these -- it means the combination is documented to potentially need dose or timing adjustment, which is exactly the kind of thing a prescriber monitors for with routine thyroid labs. Never adjust a levothyroxine dose yourself based on this page.

See levothyroxine's own medication page for its full reviewed interaction list, or Medication Safety for broader guidance.