Objective: To determine whether simultaneous ingestion of ferrous sulfate and thyroxine reduces the efficacy of thyroid hormone in patients with primary hypothyroidism.
Design: Uncontrolled clinical trial.
Setting: Outpatient research clinic of a tertiary care center.
Patients: Fourteen patients with established primary hypothyroidism on stable thyroxine replacement.
Intervention: All patients were instructed to ingest simultaneously, a 300-mg ferrous sulfate tablet and their usual thyroxine dose every day for 12 weeks.
Results: After 12 weeks of ferrous sulfate ingestion with thyroxine, the mean level of serum thyrotropin (thyroid stimulating hormone, TSH) rose from 1.6 +/- 0.4 to 5.4 +/- 2.8 mU/L (P < 0.01), but the free thyroxine index did not change significantly. Subjective evaluation using a clinical score showed that nine patients had an increase in symptoms and signs of hypothyroidism; the mean score for the 14 patients changed from 0 to 1.3 +/- 0.4 (P = 0.011). When iron and thyroxine were mixed together in vitro, a poorly soluble purple complex appeared that indicated the binding of iron to thyroxine.
Conclusions: Simultaneous ingestion of ferrous sulfate and thyroxine causes a variable reduction in thyroxine efficacy that is clinically significant in some patients. The interaction is probably caused by the binding of iron to thyroxine.