We present the case of a 60-year-old woman with long-standing primary hypothyroidism, clinically and biochemically euthyroid on the same dose of thyroxine for years, who became hypothyroid after starting daily intake of ferrous fumarate. Previously normal serum thyrotropin levels increased to 243 (reference interval: 0.32-5.00 mlU/L), and free thyroxine levels decreased to <0.52 (reference interval: 11.0-23.0 pmol/L) after oral ferrous fumarate was started. Two months after the iron supplement was discontinued, the thvroid function indices returned to normal.
Iron supplements are prescribed commonly, and over-the-counter preparations are readily available to patients. The interaction between iron and thyroxine always should be considered when both are required, especially when a patient with primary hypothyroidism becomes refractory to maintenance thyroxine doses after starting daily ingestion of iron.
(C) 1999 Lippincott Williams & Wilkins, Inc.