Essential Blood Biomarkers
A comprehensive reference for the markers that matter most — what each one measures, why it matters, and how to interpret it in the context of health optimization.
Biomarkers
Metabolic Health
Fasting Glucose
What It Measures
Blood sugar concentration after an overnight fast (8–12 hours without food).
Why It Matters
Elevated fasting glucose is a late-stage indicator of metabolic dysfunction. By the time fasting glucose is abnormal, insulin resistance has typically been present for years. Optimal fasting glucose is generally considered to be below 90 mg/dL — well below the clinical threshold for prediabetes (100–125 mg/dL).
Interpretation Overview
Optimal: <90 mg/dL. Prediabetes range: 100–125 mg/dL. Diabetes: ≥126 mg/dL on two separate tests. Reference ranges vary by lab.
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Fasting Insulin
What It Measures
Insulin concentration in the blood after an overnight fast.
Why It Matters
Fasting insulin is one of the earliest and most sensitive markers of insulin resistance — often elevated for years before fasting glucose becomes abnormal. It is not included in standard panels and must be specifically requested. Low fasting insulin with normal glucose suggests good insulin sensitivity.
Interpretation Overview
Optimal: <6 µIU/mL. Elevated: >10 µIU/mL suggests insulin resistance. Reference ranges vary by lab.
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HbA1c
What It Measures
The percentage of hemoglobin molecules with glucose attached — a 2–3 month average of blood sugar levels.
Why It Matters
HbA1c provides a longer-term view of glucose control than a single fasting glucose reading. It is used to diagnose prediabetes and type 2 diabetes, and to monitor glycemic management over time. Optimal HbA1c for health optimization is generally considered to be below 5.4%.
Interpretation Overview
Optimal: <5.4%. Prediabetes: 5.7–6.4%. Diabetes: ≥6.5%. Reference ranges vary by lab.
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C-Peptide
What It Measures
A byproduct of insulin production — released in equal amounts to insulin when the pancreas produces it.
Why It Matters
C-peptide helps distinguish between type 1 and type 2 diabetes, and can assess residual pancreatic beta cell function. In the context of health optimization, it provides additional context for insulin production that fasting insulin alone does not capture.
Interpretation Overview
Normal range: 0.5–2.0 ng/mL. Elevated C-peptide with elevated insulin suggests insulin resistance. Low C-peptide may indicate beta cell dysfunction.
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HOMA-IR
What It Measures
Homeostatic Model Assessment of Insulin Resistance — a calculated index derived from fasting glucose and fasting insulin.
Why It Matters
HOMA-IR is one of the most practical tools for quantifying insulin resistance. It is calculated as (fasting insulin × fasting glucose) / 405 (using mg/dL for glucose). A HOMA-IR above 2.0 is generally considered to indicate insulin resistance; above 2.9 is associated with significant metabolic dysfunction.
Interpretation Overview
Optimal: <1.0. Borderline: 1.0–2.0. Insulin resistance: >2.0. Significant IR: >2.9. Calculated — not a direct lab test.
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Biomarkers
Cardiovascular Health
ApoB
What It Measures
The number of atherogenic lipoprotein particles in the blood — each LDL, VLDL, IDL, and Lp(a) particle carries exactly one ApoB molecule.
Why It Matters
ApoB is a more accurate predictor of cardiovascular risk than LDL-C because it counts the actual number of particles that can penetrate the arterial wall and initiate atherosclerosis. Two people can have identical LDL-C but very different ApoB levels — and very different cardiovascular risk. Most physicians do not order it by default.
Interpretation Overview
Optimal: <80 mg/dL. Acceptable: <100 mg/dL. Elevated risk: >120 mg/dL. Reference ranges vary by lab and risk category.
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Lipoprotein(a)
What It Measures
A genetically determined lipoprotein particle — structurally similar to LDL but with an additional apolipoprotein(a) protein attached.
Why It Matters
Lp(a) is an independent, largely genetic cardiovascular risk factor that affects approximately 20% of the population. It is not meaningfully modifiable through diet or lifestyle. It is not included in standard lipid panels and must be specifically requested. Elevated Lp(a) significantly increases the risk of atherosclerosis, heart attack, and aortic stenosis.
Interpretation Overview
Optimal: <30 mg/dL (or <75 nmol/L). High risk: >50 mg/dL (or >125 nmol/L). Units vary by lab — confirm which unit your lab uses.
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hs-CRP
What It Measures
High-sensitivity C-reactive protein — a marker of systemic inflammation produced by the liver in response to inflammatory signals.
Why It Matters
Chronic low-grade inflammation is a key driver of atherosclerosis and cardiovascular events. hs-CRP is sensitive enough to detect low-level inflammation that standard CRP tests miss. Elevated hs-CRP is an independent predictor of cardiovascular risk, even in people with normal lipid panels.
Interpretation Overview
Low risk: <1.0 mg/L. Average risk: 1.0–3.0 mg/L. High risk: >3.0 mg/L. Values >10 mg/L may indicate acute infection or injury.
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Triglycerides
What It Measures
The concentration of triglyceride molecules in the blood — a form of fat used for energy storage and transport.
Why It Matters
Elevated triglycerides are strongly associated with insulin resistance, metabolic syndrome, and cardiovascular risk. They are one of the most diet-responsive markers — carbohydrate intake, alcohol, and refined sugar are primary drivers. The triglyceride-to-HDL ratio is a useful proxy for insulin resistance.
Interpretation Overview
Optimal: <100 mg/dL. Acceptable: <150 mg/dL. Borderline high: 150–199 mg/dL. High: ≥200 mg/dL.
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HDL Cholesterol
What It Measures
High-density lipoprotein cholesterol — often called "good cholesterol," though its role is more nuanced than that label suggests.
Why It Matters
HDL is involved in reverse cholesterol transport — moving cholesterol from peripheral tissues back to the liver. Low HDL is associated with increased cardiovascular risk, particularly when combined with elevated triglycerides. The triglyceride-to-HDL ratio (ideally below 2.0) is a useful metabolic health indicator.
Interpretation Overview
Low (increased risk): <40 mg/dL (men), <50 mg/dL (women). Optimal: >60 mg/dL. Very high HDL may not be protective in all contexts.
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LDL Cholesterol
What It Measures
Low-density lipoprotein cholesterol — the cholesterol carried within LDL particles.
Why It Matters
LDL-C is the most commonly measured cardiovascular risk marker, but it is an imperfect proxy for actual risk. LDL-C measures the amount of cholesterol in LDL particles, not the number of particles. ApoB is a more accurate measure of atherogenic particle burden. LDL-C remains clinically relevant but should be interpreted alongside ApoB and other markers.
Interpretation Overview
Optimal: <100 mg/dL. Near optimal: 100–129 mg/dL. Borderline high: 130–159 mg/dL. High: ≥160 mg/dL. Context-dependent — interpret with ApoB.
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Biomarkers
Liver Health
AST (Aspartate Aminotransferase)
What It Measures
An enzyme found primarily in the liver and heart muscle. Elevated levels indicate cell damage in these tissues.
Why It Matters
AST is a general marker of liver and cardiac stress. It is less liver-specific than ALT — elevated AST can reflect muscle damage, cardiac injury, or liver disease. The AST-to-ALT ratio can help distinguish between different causes of liver enzyme elevation.
Interpretation Overview
Normal range: 10–40 U/L (varies by lab). Elevated AST with elevated ALT suggests liver involvement. Isolated elevated AST may indicate muscle or cardiac origin.
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ALT (Alanine Aminotransferase)
What It Measures
An enzyme found primarily in the liver. Elevated levels are a more specific indicator of liver cell damage than AST.
Why It Matters
ALT is the most liver-specific of the standard liver enzymes. Chronically elevated ALT — even within the "normal" reference range — is associated with non-alcoholic fatty liver disease (NAFLD), insulin resistance, and increased cardiovascular risk. Optimal ALT is generally considered to be below 25 U/L for women and below 33 U/L for men.
Interpretation Overview
Optimal: <25 U/L (women), <33 U/L (men). Reference ranges vary by lab — many labs use upper limits of 40–56 U/L, which may miss early liver stress.
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GGT (Gamma-Glutamyl Transferase)
What It Measures
An enzyme involved in glutathione metabolism and found in the liver, bile ducts, kidneys, and pancreas.
Why It Matters
GGT is a sensitive marker of liver stress and oxidative burden. It is elevated by alcohol consumption, certain medications, fatty liver disease, and oxidative stress. Elevated GGT is an independent predictor of cardiovascular events and all-cause mortality, even within the "normal" reference range.
Interpretation Overview
Normal range: 9–48 U/L (varies by lab and sex). Optimal is generally considered to be in the lower portion of the reference range.
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Bilirubin
What It Measures
A yellow pigment produced from the breakdown of red blood cells, processed and excreted by the liver.
Why It Matters
Total bilirubin reflects the liver's ability to process and excrete waste products from red blood cell turnover. Elevated bilirubin can indicate liver dysfunction, bile duct obstruction, or hemolysis. Mildly elevated bilirubin (as in Gilbert's syndrome) is generally benign and may even be associated with antioxidant effects.
Interpretation Overview
Normal total bilirubin: 0.1–1.2 mg/dL. Elevated levels warrant investigation for liver or bile duct pathology.
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Biomarkers
Thyroid Health
TSH (Thyroid-Stimulating Hormone)
What It Measures
A pituitary hormone that signals the thyroid gland to produce thyroid hormones. It is the primary screening marker for thyroid function.
Why It Matters
TSH is the most sensitive indicator of thyroid function — it rises when thyroid hormone levels are low (hypothyroidism) and falls when they are high (hyperthyroidism). However, TSH alone does not tell the full story. Normal TSH with suboptimal free T3 or T4 can still produce symptoms of thyroid dysfunction.
Interpretation Overview
Standard reference range: 0.4–4.0 mIU/L. Many functional medicine practitioners consider optimal TSH to be 1.0–2.5 mIU/L. Interpret alongside free T3 and free T4.
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Free T3
What It Measures
The active, unbound form of triiodothyronine — the primary biologically active thyroid hormone at the cellular level.
Why It Matters
Free T3 is the most metabolically active thyroid hormone. It is converted from T4 in peripheral tissues and drives cellular energy production, metabolism, and thermogenesis. Low free T3 — even with normal TSH — can produce symptoms of hypothyroidism including fatigue, weight gain, and cognitive impairment.
Interpretation Overview
Normal range: 2.3–4.2 pg/mL (varies by lab). Optimal is generally considered to be in the upper half of the reference range.
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Free T4
What It Measures
The unbound, inactive form of thyroxine — the primary thyroid hormone produced by the thyroid gland, converted to T3 in peripheral tissues.
Why It Matters
Free T4 reflects thyroid gland output and the substrate available for conversion to active T3. Low free T4 with elevated TSH confirms primary hypothyroidism. Normal free T4 with low free T3 may indicate impaired peripheral conversion.
Interpretation Overview
Normal range: 0.8–1.8 ng/dL (varies by lab). Interpret alongside TSH and free T3 for a complete picture.
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Reverse T3
What It Measures
An inactive form of T3 produced when T4 is converted through an alternative pathway — it competes with active T3 at receptor sites.
Why It Matters
Elevated reverse T3 can block active T3 from binding to receptors, producing functional hypothyroidism even when TSH and free T4 appear normal. It is elevated in states of chronic stress, caloric restriction, illness, and high cortisol. The free T3-to-reverse T3 ratio is used by some practitioners as a measure of thyroid efficiency.
Interpretation Overview
Normal range: 9.2–24.1 ng/dL (varies by lab). Elevated reverse T3 with low free T3 may indicate impaired thyroid hormone utilization.
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TPO Antibodies
What It Measures
Antibodies against thyroid peroxidase — an enzyme essential for thyroid hormone synthesis.
Why It Matters
Elevated TPO antibodies are the primary marker of Hashimoto's thyroiditis, the most common cause of hypothyroidism. They can be elevated for years before TSH becomes abnormal. Testing TPO antibodies is essential for understanding the root cause of thyroid dysfunction — not just its presence.
Interpretation Overview
Normal: <35 IU/mL (varies by lab). Elevated TPO antibodies indicate autoimmune thyroid disease regardless of TSH status.
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Biomarkers
Nutrient Status
Vitamin D (25-OH)
What It Measures
25-hydroxyvitamin D — the storage form of vitamin D and the standard marker for vitamin D status.
Why It Matters
Vitamin D functions as a hormone, with receptors in virtually every tissue in the body. Deficiency is associated with immune dysfunction, cardiovascular disease, metabolic syndrome, cognitive decline, and increased all-cause mortality. Most people in northern latitudes are deficient, and the standard reference range lower limit (20 ng/mL) is widely considered insufficient for optimal health.
Interpretation Overview
Deficient: <20 ng/mL. Insufficient: 20–29 ng/mL. Optimal: 40–60 ng/mL (functional medicine consensus). Toxicity risk: >100 ng/mL.
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Vitamin B12
What It Measures
Serum cobalamin — a water-soluble vitamin essential for neurological function, DNA synthesis, and red blood cell formation.
Why It Matters
B12 deficiency causes neurological damage, megaloblastic anemia, and cognitive impairment. It is common in people following plant-based diets, older adults (due to reduced gastric acid), and those taking metformin or proton pump inhibitors. Serum B12 can appear normal while functional deficiency exists — methylmalonic acid and homocysteine provide additional context.
Interpretation Overview
Normal range: 200–900 pg/mL (varies by lab). Optimal is generally considered >500 pg/mL. Deficiency: <200 pg/mL.
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Folate
What It Measures
Serum or red blood cell folate — a B vitamin essential for DNA synthesis, methylation, and homocysteine metabolism.
Why It Matters
Folate is critical for one-carbon metabolism and methylation reactions throughout the body. Deficiency impairs DNA repair, raises homocysteine, and increases cardiovascular and neurological risk. MTHFR gene variants affect folate metabolism and may require specific forms of supplementation (methylfolate rather than folic acid).
Interpretation Overview
Normal serum folate: 2.7–17.0 ng/mL (varies by lab). RBC folate provides a longer-term measure. Interpret alongside homocysteine and B12.
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Magnesium
What It Measures
Serum magnesium concentration — though serum levels reflect only ~1% of total body magnesium, with the majority stored intracellularly.
Why It Matters
Magnesium is a cofactor in over 300 enzymatic reactions, including those involved in insulin signaling, glucose metabolism, ATP production, and DNA repair. Serum magnesium is an insensitive marker of deficiency — RBC magnesium provides a better picture of intracellular status. Deficiency is common in people eating a modern diet and is associated with insulin resistance, hypertension, and cardiovascular disease.
Interpretation Overview
Normal serum range: 1.7–2.2 mg/dL. RBC magnesium optimal: 5.2–6.5 mg/dL. Serum levels can appear normal despite intracellular deficiency.
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Zinc
What It Measures
Serum zinc concentration — an essential trace mineral involved in immune function, protein synthesis, and enzymatic activity.
Why It Matters
Zinc is required for over 300 enzymes and plays a central role in immune defense, wound healing, testosterone production, and antioxidant activity. Deficiency impairs immune function, reduces testosterone, and is associated with increased oxidative stress. Like magnesium, serum zinc is an imperfect marker of total body status.
Interpretation Overview
Normal range: 60–120 µg/dL (varies by lab). Deficiency: <60 µg/dL. Interpret in the context of symptoms and dietary intake.
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Ferritin
What It Measures
A protein that stores iron — serum ferritin reflects total body iron stores and is also an acute-phase reactant that rises with inflammation.
Why It Matters
Ferritin is the most sensitive marker of iron status. Low ferritin indicates iron depletion before anemia develops. Elevated ferritin can indicate iron overload (hemochromatosis), but also reflects chronic inflammation, metabolic syndrome, and liver disease — making it a useful metabolic health marker beyond its role in iron assessment.
Interpretation Overview
Iron deficiency: <12 ng/mL. Optimal (men): 50–150 ng/mL. Optimal (women): 30–100 ng/mL. Elevated (>200 ng/mL) warrants investigation for iron overload or inflammation.
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Disclaimer: Interpretation ranges provided are for educational purposes only and represent general guidance from published literature. Reference ranges vary by laboratory, methodology, and individual context. Always consult a qualified healthcare professional to interpret your specific results and make clinical decisions.