What a Low Hemoglobin Level Means

Hemoglobin is the iron-containing protein inside red blood cells that picks up oxygen in the lungs and releases it around the body. The test measures how much of it is in a given volume of blood, usually reported in grams per deciliter (gm/dL).

It is almost always run as part of a complete blood count, or CBC — one of the most common blood tests, often done as part of a routine checkup, which also measures red blood cells, white blood cells, platelets, hematocrit and mean corpuscular volume (National Heart, Lung, and Blood Institute, 2025-12-18). The specimen is blood, usually drawn from a vein in the arm; a fingerstick is sometimes used for screening.

Doctors order it to check for anemia — a condition that develops when the blood produces a lower-than-normal amount of healthy red blood cells, so the body does not get enough oxygen-rich blood, which can cause tiredness or weakness, shortness of breath, dizziness, headaches or an irregular heartbeat (NHLBI, 2022-03-24). It is also ordered to follow a known condition, before surgery, or during pregnancy.

Fasting is not required. Timing matters in a different way: hydration affects the result, because hemoglobin is a concentration. Heavy fluid loss can push it up and extra fluid, including IV fluids, can dilute it downward. That is one reason a single reading is read alongside the rest of the CBC rather than on its own.

Use the range printed on your own report, not this table. Normal value ranges vary slightly among laboratories, and some labs use different measurements or test different samples (MedlinePlus, 2024-10-14). The NHLBI gives a normal adult range of 14 to 17 gm/dL for men and 12 to 15 gm/dL for women and notes the range varies with altitude (NHLBI, 2025-12-18); ranges can also differ with age and race (NHLBI, 2025-12-18). A value slightly outside the printed range means something different from one far outside it, and only your clinician can place yours in context.

All of these push hemoglobin down. They are listed roughly from the most routine to the less common.

• Iron deficiency. The most common type of anemia is iron-deficiency anemia (NHLBI, 2022-03-24). Too little iron means too little hemoglobin can be built.
• Blood loss. Heavy menstrual periods, bleeding in the stomach or bowel, recent surgery or injury all lower the count, sometimes before you notice symptoms.
• Diet low in key nutrients. Too little iron, folate, vitamin B12 or vitamin B6 from poor nutrition is a recognised cause of low hemoglobin (MedlinePlus, 2024-10-14).
• Dilution. Pregnancy and IV fluids increase blood volume, which lowers the measured concentration without a change in total hemoglobin.
• Chronic conditions. Chronic kidney disease, ulcerative colitis and rheumatoid arthritis are among the long-standing conditions associated with lower counts (MedlinePlus, 2024-10-14).
• Long-term infection. Ongoing infections such as hepatitis can lower the number (MedlinePlus, 2024-10-14).
• Red blood cells breaking down faster than normal (hemolysis). Less common than the causes above, and generally identified with extra blood tests.
• Bone marrow problems and blood cancers. Bone marrow failure, leukemia and multiple myeloma are listed causes (MedlinePlus, 2024-10-14), but they are uncommon compared with iron deficiency and blood loss. Anemia may also be a sign of a more serious condition such as bleeding in the stomach, inflammation from infection, kidney disease, cancer or autoimmune disease (NHLBI, 2022-03-24).

1. Read the result against your own lab's range. Note the figure in gm/dL, the printed range, and whether the lab flagged it. Check whether earlier CBCs show the same number drifting or a sudden change.
2. Decide on timing. It is reasonable to wait for the scheduled appointment if you feel as you normally do. Contact your doctor sooner — or go to an emergency department — if you have chest pain, fainting or near-fainting, shortness of breath at rest, a racing or irregular heartbeat, black tarry stools, blood in stool or vomit, or heavy vaginal bleeding. Mild anemia may cause no symptoms at all (NHLBI, 2022-03-24), so the absence of symptoms does not settle the question either way.
3. Expect a repeat or confirmatory test. A single value can be affected by hydration or sample handling, so clinicians commonly repeat the CBC and add tests such as iron studies or vitamin levels.
4. Bring the printed report. Take the lab slip, portal printout or after-visit summary to the appointment, including the reference range and any prior results, rather than quoting the number from memory.
5. Work through the cause. To diagnose anemia a doctor may ask about risk factors, medical history, diet and family history, examine for signs such as a pale tongue or brittle nails, and order further tests; if anemia is found, you may be referred to a hematologist, a doctor who specialises in blood diseases (NHLBI, 2025-12-18).
6. Further testing if the source is unclear. A colonoscopy may be ordered to look for bleeding or tumors in the colon, or an endoscopy to look for bleeding in the esophagus, stomach and first part of the small intestine (NHLBI, 2025-12-18). Bone marrow aspiration and biopsy can help find the cause of low blood counts and may be done at a hospital, surgery center or sometimes a clinic (NHLBI, 2025-12-18).