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What are the causes of elevated HbF levels?

Foetal hemoglobin is formed by 2 alpha subunits and two gamma subunits  and their four heme groups.
Normal level of fetal hemoglobin
Fetal hemoglobin (hemoglobin F, HbF) is the major hemoglobin that is present during gestation; it constitutes about 60 to 80 percent of total hemoglobin in a full-term newborn. This is almost completely replaced by adult hemoglobin (hemoglobin A, HbA) by 6 to 12 months of age.
Hemoglobin F amounts to less than 1 percent of total hemoglobin in the adult.

Causes of elevated HbF
Haemoglobinopathies with elevated  HbF levels
  • Beta thalassemia major
  • Beta  thalassemia minor
  • Sickle cell disease
Anemia with elevated  HbF levels
  • Non Hereditary refractory normoblastic anemia
  • Pernicious anemia
  • Aplastic anemia
Hereditary Persistence of HbF
Leukemia specially Juvenile Myeloid Leukemia
Multiple myeloma
Molar pregnancy
Patients with Trisomy 13 or Trisomy 21 (Down’s syndrome)
Certain  chronic viral infections (eg CMV, EBV)
Clinical significance of Hemoglobin F 
In newborns, foetal hemoglobin is almost completely replaced by adult hemoglobin by the age of  6 months postnatally, but in a few thalassemia patients there may be a delay in cessation of HbF production until 3–5 years of age.
Foetal hemoglobin differs from adult hemoglobin in that it has more affinity to oxygen  affinity than the adult form, so better availability of oxygen to fetus from the mother's bloodstream.
As it is a minor hemoglobin in the normal child and adult, HbF has little clinical relevance in normal physiology. But it is assuming greatest importance in certain hemoglobinopathies where an increases in HbF have been shown to improve the clinical performance of affected individuals with sickle cell disease and beta thalassemia. In adults, foetal hemoglobin production can be reactivated pharmacologically which is useful in the treatment of Thalassemia and  sickle-cell disease