You just got your blood work back. Most people scan the page looking for those little "H" or "L" flags next to the numbers. If you’re seeing a high reading for hemoglobin (HGB) or hematocrit (HCT), your first instinct might be to panic. It’s a weird feeling, right? Finding out there’s "too much" of something that is supposed to be good for you. Hemoglobin is the protein in your red blood cells that carries oxygen. Hematocrit is basically the percentage of your blood that consists of those red cells. When these numbers climb, it basically means your blood is becoming more concentrated—kinda like a soup that’s been simmering on the stove too long and has gotten a bit too thick.
High HGB and HCT aren't always a sign of a scary disease. Sometimes, you're just thirsty. Other times, your body is overcompensating for something else entirely. We need to look at the "why" because "thick blood" isn't a diagnosis; it’s a symptom.
What Causes High HGB and HCT in Everyday Life?
The most common reason for a spike in these numbers is actually pretty boring: dehydration. If you haven't been drinking enough water, or if you've been sweating through a workout without replenishing, the liquid part of your blood (plasma) drops. When plasma drops, the concentration of red blood cells naturally looks higher on a lab test. It's a false elevation. You don't actually have more red blood cells; you just have less water. Drink a few big glasses of water, wait a day, and those numbers usually settle right back down to where they belong.
Then there’s the "where" of your life. Geography matters. If you recently moved from the coast up to Denver or some other high-altitude spot, your HGB and HCT are going to climb. Why? Because there's less oxygen up there. Your kidneys sense the thin air and start pumping out a hormone called erythropoietin (EPO). This hormone tells your bone marrow, "Hey, we need more red blood cells to catch what little oxygen we have!" It’s a brilliant survival mechanism. Mountaineers and elite athletes often have naturally high HGB and HCT levels because their bodies have adapted to low-oxygen environments.
Smoking is another huge one. This is something people often overlook. When you inhale cigarette smoke, you’re taking in carbon monoxide. That carbon monoxide binds to your hemoglobin much more tightly than oxygen does. It basically "hijacks" the red blood cell, making it useless for oxygen transport. Your body, sensing an oxygen shortage, produces more red blood cells to make up for the ones being occupied by the smoke. If you're a heavy smoker, your HCT might consistently sit at the high end of the range or just above it. It's your body's way of trying not to suffocate at a cellular level.
The Role of Lung and Heart Health
Sometimes the cause is internal. Chronic obstructive pulmonary disease (COPD) or severe sleep apnea can keep your oxygen levels low for hours at a time. If you’re snoring heavily or stopping breathing in your sleep, your body spends all night in a state of oxygen deprivation. Just like the mountain climber, your bone marrow starts overproducing red blood cells to compensate. Doctors often see high HCT in patients with undiagnosed sleep apnea. It's a massive red flag.
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Heart failure can also lead to this. If the heart isn't pumping blood effectively, the tissues don't get enough oxygen. The kidneys, acting as the body's sensors, respond by triggering more red blood cell production. It’s a well-intentioned response from the body that can actually make things worse, as thicker blood is harder for a weak heart to pump. It’s a vicious cycle.
Polycythemia Vera and Primary Causes
Now, we have to talk about the more serious stuff. Primary polycythemia, specifically a condition called Polycythemia Vera (PV), is a slow-growing blood cancer. It sounds terrifying, but it's often manageable. In PV, a mutation in the JAK2 gene causes your bone marrow to go into overdrive. It starts churning out red blood cells even when your body doesn't need them.
Unlike the "secondary" causes we talked about—like smoking or altitude—this isn't a compensation. It's a factory error. People with PV often deal with intense itching, especially after a warm bath (aquagenic pruritus), and a weird redness in their face or palms. Their HGB and HCT levels can reach levels that significantly increase the risk of blood clots or strokes because the blood becomes truly "sludge-like."
Expert Note: According to the Leukemia & Lymphoma Society, Polycythemia Vera is rare, affecting about 44 to 57 people per 100,000. It's not the first thing a doctor suspects, but it's always on the radar if other causes are ruled out.
Performance Enhancers and Medications
We can't ignore the elephant in the room: supplements and medications. Testosterone Replacement Therapy (TRT) is a very common cause of high HGB and HCT in men. Testosterone directly stimulates the bone marrow to produce more red blood cells. If you’re on TRT and your HCT hits 54% or higher, most doctors will recommend adjusting your dose or donating blood to thin things out.
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Then there are the "performance enhancers." Epoetin alfa (often known by the brand name Procrit or Epogen) is used medically to treat anemia, but it’s also been used illicitly by endurance athletes for blood-doping. It artificially inflates the red blood cell count to boost stamina. If someone is using these substances, their blood work will show a massive spike in HGB and HCT. It’s dangerous. When your hematocrit gets too high—a condition called hyperviscosity—the risk of a heart attack or pulmonary embolism skyrockets.
Understanding the Risks of "Thick Blood"
Why do doctors care so much about these numbers? It’s all about flow. Your blood needs to be thin enough to zip through tiny capillaries but thick enough to carry nutrients. When HCT climbs too high, the viscosity of the blood increases. This puts a strain on your vascular system.
It's like trying to pump maple syrup through a straw instead of water. You can do it, but you have to suck a lot harder. Your heart has to work harder. Your blood pressure might go up. Most importantly, slow-moving blood is more likely to form a clot. This is why high HCT is often linked to an increased risk of Deep Vein Thrombosis (DVT) or even a stroke.
Actionable Steps: What Should You Do Now?
If you're looking at a lab report with high numbers, don't jump to the worst-case scenario. Most of the time, the fix is simpler than you think.
Hydrate like it's your job.
Before you do anything else, increase your water intake for 48 hours. Avoid excessive caffeine or alcohol, which act as diuretics. Ask your doctor for a re-test after you’re fully hydrated. If the numbers drop, you’ve found your culprit.
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Check your lifestyle habits.
Are you a smoker? Do you live at high altitude? Do you use testosterone or other hormones? These are the "low hanging fruit" of high blood counts. If you suspect sleep apnea—maybe your partner says you gasp for air at night—get a sleep study. Treating the apnea often brings the HCT right back to normal.
Ask for a "Differential" or more specific tests.
If hydration doesn't fix it, your doctor needs to dig deeper. They might check your EPO levels.
- Low EPO often points toward Polycythemia Vera (because the marrow is making cells without being told to).
- High EPO usually means your body is reacting to low oxygen (smoking, lung disease, altitude).
The JAK2 Mutation Test.
If the doctor suspects a bone marrow issue, they will run a genetic test for the JAK2 mutation. This is a definitive way to screen for Polycythemia Vera. It’s a simple blood draw, not a bone marrow biopsy (initially).
Consider therapeutic phlebotomy.
This is a fancy way of saying "giving blood." For many people with chronically high HCT—whether from TRT or PV—donating a pint of blood every few months is the standard treatment. It’s effective, fast, and literally thins out the blood volume.
Monitor for symptoms.
Watch out for "red flag" symptoms like blurred vision, severe headaches, dizziness, or a heavy feeling in your chest. If your blood is thick and you start feeling these things, it’s not a "wait and see" situation. It's a "call the doctor today" situation.
In most cases, high HGB and HCT are manageable. Whether it's drinking more water, quitting the cigarettes, or managing a chronic condition, you have options. The numbers on the page are just a map; they tell you where you are, but they don't have to be where you stay. Talk to your healthcare provider about these specific causes and see which one fits your life. Focus on the underlying cause, not just the number.