Vitamin B12 or Folate Deficiency: This is the most common cause of high Mean Corpuscular Volume (MCV) with normal hemoglobin levels. Deficiencies in either vitamin B12 or folate can lead to impaired DNA synthesis, resulting in the production of larger than normal red blood cells (megaloblasts), which increases the MCV.

Alcoholism: Chronic alcohol consumption can lead to a direct toxic effect on the bone marrow, causing an increase in MCV. Alcohol also interferes with folate metabolism, further contributing to the development of macrocytosis.
Hypothyroidism: Although less common, hypothyroidism can cause an increase in MCV, possibly due to decreased metabolism and alterations in lipid metabolism affecting the red blood cell membrane.
Liver Disease: Certain liver diseases, such as cirrhosis, can lead to an increase in MCV due to alterations in lipid metabolism and the production of abnormal lipoproteins that affect red blood cell membranes.

Myeloproliferative Neoplasms (e.g., Polycythemia Vera, Essential Thrombocytosis): While these conditions more commonly present with an increase in red blood cell mass, some cases can have a high MCV. Missing these diagnoses could lead to significant morbidity and mortality due to the risk of thrombosis and transformation to acute myeloid leukemia.
Myelodysplastic Syndromes: These are a group of disorders caused by poorly formed or dysfunctional blood cells, which can sometimes present with macrocytosis. Early detection is crucial as these syndromes can progress to acute leukemia.

Orotic Aciduria: A rare genetic disorder affecting pyrimidine synthesis, leading to megaloblastic anemia with a high MCV.
Congenital Dyserythropoietic Anemia: A group of rare genetic disorders characterized by ineffective erythropoiesis and often macrocytosis.
Pharmaceutical Causes: Certain medications, such as hydroxyurea, azathioprine, and some anticonvulsants, can cause macrocytosis as a side effect. The likelihood depends on the specific medication and the duration of treatment.