How does methemoglobin occur?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Methemoglobin contains iron in the ferric state (Fe3+) rather than the reduced ferrous form (Fe2+) found in hemoglobin. This structural change causes an alteration in the blood’s ability to bind oxygen. Methemoglobin is a naturally occurring oxidized metabolite of hemoglobin, and physiologic levels (< 1%) are normal. Problems arise as methemoglobin levels increase. Methemoglobin does not bind oxygen, thus effectively leading to a functional anemia. [1, 2, 3, 4]

In addition, methemoglobin causes a leftward shift of the oxygen-hemoglobin dissociation curve, resulting in decreased release of oxygen to the tissues. The presence of anemia and cyanosis despite oxygen treatment results from both of these effects. [4, 5] (See Pathophysiology and Etiology.)

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