levothyroxine and ferrous sulfate

Ferrous sulfate chelates with levothyroxine in the gastrointestinal tract, forming an insoluble complex that reduces absorption of levothyroxine. This is a pharmacokinetic interaction at the absorption level, not involving specific enzymes or transporters.

Reduced levothyroxine absorption can lead to decreased serum thyroid hormone levels and elevated TSH, resulting in hypothyroid symptoms such as fatigue, weight gain, cold intolerance, and worsening of thyroid function. The effect may be clinically significant if doses are not adjusted or separated appropriately.

Separate administration of levothyroxine and ferrous sulfate by at least 4 hours. Monitor serum TSH and free T4 levels, especially when starting or stopping iron therapy, and adjust the levothyroxine dose as needed. If separation is not feasible, consider alternative iron preparations or routes, and reassess thyroid function regularly.

This interaction is well established through clinical studies and case reports, and is recognised in standard drug interaction resources and thyroid disease management guidelines.