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Both thyroid and iron supplements are among the most commonly prescribed medications. This study suggests that ferrous sulfate reduces thyroxine absorption when the two are simultaneously ingested.
Fourteen hypothyroid patients were asked to combine their daily thyroxine dose with a 300-mg tablet of ferrous sulfate. After 12 weeks, the mean thyroid-stimulating hormone (TSH) level rose significantly from 1.6 to 5.4 mU/l (the normal range in this laboratory is 0.5 to 4.0 mU/l). Serum TSH rose in 11 of the 14 patients. The free thyroxine index fell, although not significantly, in 10 patients.
In a related experiment, the researchers mixed thyroxine and ferrous sulfate in vitro. The compounds formed a poorly soluble complex, indicating that the i…