A low hemoglobin count, also known as anemia, is caused by pregnancy, a menstrual period, a diet low in iron, blood loss, or a disease that causes the body to produce fewer red blood cells or to destroy red blood cells. According to the Mayo Clinic, medications also sometimes cause anemia. These include antiretroviral drugs for HIV and chemotherapy drugs for cancer.
Hemoglobin carries oxygen through the blood vessels to provide it to the body's organs. Hemoglobin levels are checked with a blood test. Normal hemoglobin counts are above 13.5 grams of hemoglobin per deciliter of blood in men and 12 grams per deciliter of blood for women, according to the Mayo Clinic.
Diseases that cause the body to produce fewer red blood cells include aplastic anemia, cancer, cirrhosis of the liver, Hodgkin's disease, hypothyroidism, iron deficiency anemia, kidney disease, lead poisoning, leukemia, multiple myeloma, myelodysplastic syndromes, non-Hodgkin's lymphoma and vitamin deficiency anemia. Diseases that cause a body to destroy red blood cells include an enlarged spleen, porphyria, sickle cell anemia, thalassemia and vasculitis.
Anemia has other potential causes. Substantial bleeding, recurrent blood donations and menstrual periods also sometimes lead to anemia, according to Mayo Clinic. Pregnancy sometimes triggers anemia because the body is producing additional blood, and the woman's diet is sometimes lacking sufficient iron to produce the extra red blood cells.