levothyroxine and ferrous fumarate

ModerateUsually manageable, but it must be recognised and monitored.

Mechanism

Pharmacokinetic interaction. Ferrous fumarate, like other iron salts, forms an insoluble complex with levothyroxine in the gastrointestinal tract, thereby reducing its absorption and decreasing its bioavailability.

What it means clinically

Reduced levothyroxine absorption can lead to suboptimal thyroid hormone levels, resulting in worsening of hypothyroidism. This may manifest as an elevated TSH and symptoms such as fatigue, weight gain, cold intolerance, constipation, and dry skin. Conversely, if iron is discontinued, levothyroxine absorption may increase, potentially causing symptoms of hyperthyroidism.

What to do

Administer levothyroxine and ferrous fumarate at least 4 hours apart to minimise the interaction. Monitor serum TSH and thyroid function tests, particularly when starting or stopping iron therapy, and adjust the levothyroxine dose as necessary. If separation is not feasible, consider alternative iron formulations or consult a specialist.

Evidence

This interaction is well established and documented in clinical guidelines, product monographs, and numerous clinical studies. It is consistently reported as a clinically significant drug interaction affecting thyroid hormone replacement.

The two drugs

levothyroxineThyroid hormoneRead the monograph ferrous fumarateiron supplementRead the monograph

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Other interactions involving levothyroxine or ferrous fumarate

Educational reference for students and clinicians in training. It does not replace professional judgement, local protocols or current prescribing information.