methemoglobin: meaning, definition, pronunciation and examples

Yes, in any significant quantity. While tiny amounts exist normally (<1%), levels above 10-15% cause clinically significant hypoxia (cyanosis, shortness of breath). Levels above 50-70% can be fatal.

It can be congenital (due to enzyme deficiencies like cytochrome b5 reductase deficiency) or acquired. Acquired causes include exposure to certain drugs (e.g., dapsone, local anaesthetics like benzocaine), nitrates in water/food, and industrial chemicals (aniline dyes).

For severe cases, the standard treatment is intravenous methylene blue, which helps convert methemoglobin back to normal hemoglobin. Milder cases may only require removal of the causative agent and supportive oxygen. Methylene blue is ineffective in congenital forms and in patients with G6PD deficiency.

Both impair oxygen transport. In carbon monoxide poisoning, CO binds tightly to the ferrous iron in hemoglobin, forming carboxyhemoglobin. In methemoglobinemia, the iron itself is oxidized to the ferric state (Fe³⁺), changing the hemoglobin molecule's structure and function.

A form of hemoglobin in which the iron has been oxidized, making it incapable of carrying oxygen effectively.

Methemoglobin is usually highly technical/scientific in register.

Methemoglobin: in British English it is pronounced /ˌmɛtˌhiːmə(ʊ)ˈɡləʊbɪn/, and in American English it is pronounced /ˌmɛtˌhiməˈɡloʊbɪn/. Tap the audio buttons above to hear it.