Why would hgb f levels be high in an adult?

Several causes: Slight increases in Hemoglobin F concentration are usually of no significance in an otherwise healthy adult and in certain ethnic/racial populations. Can otherwise occur in adults due to such acquired conditions as pregnancy, various anemias, and leukemia or inherited conditions such as one of the many forms of thalassemia and sickle cell disease. Usually a hematologist can determine the cause.