HbH questions

[chromuffin]

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So Beta thal major and Sickle Cell Disease cause massive erythroid hyperplasia. I'm assuming this is particularly due to the extravascular hemolysis produced from the RBC membrane problems. My question is.. why doesn't HbH cause this? It has RBC membrane damage.. is there something fundamental I'm not understanding about erythroid hyperplasia and extramedullary hematopoiesis?