What Causes Hemoglobin to Be Low?

Cardio Renal Foundation is a 501(c)(3) tax-exempt charity organization. Your donation advances our mission to provide a solution beyond dialysis.

A low hemoglobin is a sentence with many possible authors — and the author's identity matters more than the number, because hemoglobin itself is never the disease. If you are still on the first question — what hemoglobin does and what a low reading feels like — start with our companion page, what does low hemoglobin mean?, and come back here for the why. The Kidney Foundation's booklet puts the usual anemia line at below 13.5 g/dL for men and below 12.0 for women. Below that line, every cause in medicine files into exactly three doors, and MedlinePlus teaches them the same way your team thinks them: blood is being lost, blood is being made too slowly or with missing ingredients, or blood cells are being destroyed too soon. Walk the three doors, and add a fourth small one — dilution — at the end. As always: a list of causes is not a diagnosis of you. It is the map, and for this number the map has a red-flag gate in the first door that outranks everything else on the page.

Door One: Blood Is Being Lost

The commonest story, and the one hunted first in older adults. Blood loss depletes red cells directly — and drains the iron needed to replace them, which is how the first two doors connect. Slow bleeding in the stomach or intestines is the quiet version: an ulcer, inflamed stomach lining, colon polyps, hemorrhoids at the minor end — and, at the end nobody wants to miss, a growth in the bowel, which is precisely why new iron-deficiency anemia in an older adult earns a look inside the gut rather than a shrug and a supplement. Medicines can hold the door open: the anti-inflammatory pain relievers and the blood thinners many of our readers take both make the gut likelier to bleed — one more reason no one on this page adjusts a dose alone. Sudden heavy blood loss — from an injury, a surgery, or a bleed that declares itself — is the same door moving fast. Heavy menstrual bleeding belongs on the complete list, though it explains fewer cases in this page's readership, and frequent blood donation or heavy lab draws add their smaller drafts. However it leaves, lost blood is lost hemoglobin — and the red flags are in this door: black or tarry stools, blood in the stool, or vomit that looks like blood or coffee grounds mean the loss is happening now.

Door Two: the Factory Is Missing Ingredients

Red cells are manufactured in the bone marrow, and the line stops when the supplies do. Iron comes first: hemoglobin is built around it, iron deficiency is the most common cause of anemia overall, and MedlinePlus lists it first among the causes for a reason. In a young woman the explanation is often menstrual; in an older adult, iron deficiency is treated as a question — where is it going? — because the answer so often lives in door one. (One discipline for this page, stated plainly: iron is discussed here as a cause category only. Whether and how to replace it is a doctor's decision, and blind iron can mask the very source that needs finding.) Vitamin B12 is the second supply: without it, the marrow releases large, fragile, unfinished cells. It goes missing through poor intake, through absorption failures — pernicious anemia, where the body cannot absorb B12 at all, is the named classic — and after stomach or bowel surgery. Folate completes the trio, arriving through diet and drained by heavy alcohol use. The pattern on the lab report helps sort this door: small pale cells point toward iron, oversized cells toward B12 and folate — which is why your team reads the indices beside the hemoglobin, not the hemoglobin alone.

Door Three: the Factory Is Being Slowed — or the Product Destroyed

The third door is the one this Foundation exists beside. The kidneys make erythropoietin — EPO — the hormone that signals the marrow to build red cells (MedlinePlus states the mechanism in one line). Damaged kidneys underproduce the signal, the factory idles, and hemoglobin drifts down: the Kidney Foundation's materials name this EPO shortage the most common cause of anemia in kidney disease, deepening as function falls. Around it stands inflammation: MedlinePlus lists the long-term diseases — chronic kidney disease, cancer, ulcerative colitis, rheumatoid arthritis — whose inflammatory state both suppresses the marrow and locks iron away where it cannot be used, the anemia of chronic disease. The marrow itself can be the problem — the disorders with names like myelodysplasia, leukemia, lymphoma, and myeloma, uncommon and serious, sorted by specialists when the pattern points there, especially if white cells or platelets are low alongside. And the destruction wing of this door: hemolysis, red cells broken down early — by immune attack, by certain medicines, by inherited conditions like sickle cell disease and thalassemia that most carriers have known since youth. A small fourth door completes the map: dilution — in heavy fluid overload, the same red cells swim in more water and the concentration reads lower than the true supply. Your team reads a hemoglobin beside the ankles and the scale for exactly this reason.

How Fast Does This Need Eyes?

Sort by company and by speed. The red-flag gate first: black or tarry stools, blood in the stool, vomiting blood or coffee-ground material — urgent care or 911, not a recheck. So are chest pain, breathlessness at rest, a racing heart that won't settle, or fainting. A hemoglobin that has dropped noticeably since the last test, even without symptoms, earns a call within days — falls have authors, and fast falls have fast ones. A mild, long-stable low reading in a reader with known kidney disease is the prompt-appointment version: expected territory, still worth its scheduled hunting, because even in kidney disease the cause is verified, not assumed. Whatever the tempo, expect the first tests to be the sorting ones: iron studies, B12 and folate, a reticulocyte count to see whether the factory is answering — the map made measurable.

What to Ask Your Doctor

Four questions match the doors. Am I losing blood — and if my iron is low, where is it going; does my gut need a look? Which ingredients am I missing — what do my iron, B12, and folate say? Is this my kidneys' hormone signal, my inflammation, or the marrow itself — and is the reticulocyte count showing the factory trying? And how far and how fast has it fallen — what is my trend, and when do we measure again? A hemoglobin that gets those four answers has been properly heard.

Why This Research Matters

A low hemoglobin in kidney disease is the two organs' oldest conversation: kidneys that can no longer send the build signal, a heart straining to move thinner blood, each making the other's work harder. The Cardio Renal Foundation supports research toward a solution beyond dialysis — research aimed at the whole of what failing kidneys stop doing, the cleaning and the hormone work alike, and at easing the circle that anemia tightens between kidney and heart. That is a direction, not a promise — but it is aimed squarely at the burden this number represents.

So — what causes hemoglobin to be low? Blood lost, ingredients missing, a factory slowed by quiet kidneys or inflammation, or cells destroyed too soon. The next question — what can actually be done about it — has its own page: how can I raise my hemoglobin? Start with the three doors, respect the red-flag gate, and remember the page's first sentence: the author matters more than the number.

At the Cardio Renal Foundation (Cardio Renal Foundation Inc, EIN 33-4359765), we publish education for exactly this reader — lab report in hand, asking why. Support from readers like you helps us keep publishing it.

This article is informational only and is not meant to replace the advice of your qualified healthcare provider. Black or tarry stools, blood in the stool, vomiting blood, chest pain, breathlessness at rest, or fainting can signal an emergency: call 911 immediately. Nothing here diagnoses any condition or recommends any treatment, including any supplement or medicine. Never start, stop, or change a prescribed medicine — including a blood thinner or pain reliever discussed on this page — without speaking to the clinician who prescribed it. Research supported by this Foundation is investigational only. As with any issues related to your health, check with your doctor.