Biomarker library
A UK reference library of 35 blood biomarkers. Every entry lists the clinical (NHS) range and the tighter performance-optimal range, the symptoms of low and high levels, food sources, and a peer-reviewed citation.
35 markers across 12categories · Last reviewed June 2026
“A blood test is a conversation with your biology.
Every marker is a sentence — read them together and the story emerges.”
Vitamins & Minerals
7 markers
Vitamin D (25-hydroxyvitamin D) is a fat-soluble hormone precursor synthesised in the skin upon UVB exposure and obtained through diet. It plays a critical role in calcium absorption, immune regulation, and gene expression across hundreds of pathways. Blood levels of 25-OH-D are the gold standard for assessing vitamin D status.
UK reference range: 30-100 nmol/L (nmol/L).
Vitamin B12 is a water-soluble vitamin essential for DNA synthesis, red blood cell formation, and neurological function. It is exclusively found in animal-derived foods, making deficiency more common in vegetarians and vegans. B12 is stored in the liver, so deficiency can take years to develop but causes significant damage when it does.
UK reference range: 197-771 pg/mL (pg/mL).
Magnesium is an essential mineral involved in over 300 enzymatic reactions in the body, including energy production, protein synthesis, muscle and nerve function, and blood pressure regulation. Serum magnesium reflects only about 1% of total body stores, as most magnesium resides in bones and soft tissues — making subclinical deficiency difficult to detect.
UK reference range: 0.7-1.0 mmol/L (mmol/L).
Iron is an essential mineral that carries oxygen in your blood, supports energy production in every cell, and plays a key role in immune function and cognitive performance. A full iron panel — serum iron, total iron-binding capacity (TIBC), and transferrin saturation — gives a far more complete picture than ferritin alone. Together, these markers reveal whether your body is absorbing, transporting, and utilising iron effectively.
UK reference range: 10-30 µmol/L (serum iron) (µmol/L / %).
Folate (vitamin B9) is a water-soluble B vitamin essential for DNA synthesis, methylation, and red blood cell formation. Serum folate reflects recent dietary intake (1-3 weeks), while red blood cell folate reflects longer-term status (3-4 months). Folate works synergistically with vitamin B12 — deficiency in either can cause megaloblastic anaemia.
UK reference range: >7 nmol/L (nmol/L).
Vitamin B6 (pyridoxal 5'-phosphate in its active form) is a water-soluble vitamin involved in over 150 enzymatic reactions. It is critical for amino acid metabolism, neurotransmitter synthesis (serotonin, dopamine, GABA), haemoglobin formation, and immune function. Unlike fat-soluble vitamins, B6 is not stored in significant quantities and must be replenished regularly through diet.
UK reference range: 20-125 nmol/L (nmol/L).
Zinc is an essential trace mineral involved in over 300 enzymatic reactions. It plays critical roles in immune function, protein synthesis, wound healing, DNA synthesis, and cell division. Unlike iron or vitamin D, the body has no dedicated zinc storage mechanism — daily intake is required to maintain adequate levels. Serum zinc reflects recent intake rather than long-term stores.
UK reference range: 11-24 µmol/L (µmol/L).
Hormones
7 markers
Total testosterone measures the combined amount of bound and unbound testosterone in the blood. It is the primary male sex hormone responsible for muscle mass, bone density, red blood cell production, and reproductive function. Testosterone levels naturally peak in a man's late twenties and decline approximately 1-2% per year thereafter.
UK reference range: 8.64-29 nmol/L (nmol/L).
DHEA-S (dehydroepiandrosterone sulphate) is the most abundant steroid hormone in the body, produced primarily by the adrenal glands. It serves as a precursor to both testosterone and oestrogen. DHEA-S levels peak in the mid-20s and decline steadily at roughly 2-3% per year, making it one of the most reliable biomarkers of biological ageing.
UK reference range: 4.34-12.2 µmol/L (male) (µmol/L).
SHBG is a glycoprotein produced primarily by the liver that binds to sex hormones — testosterone, dihydrotestosterone (DHT), and oestradiol — and regulates their bioavailability. Only the unbound (free) fraction of testosterone is biologically active, so SHBG levels directly determine how much testosterone your body can actually use. SHBG is influenced by thyroid function, insulin levels, liver health, and body composition.
UK reference range: 18.3-54.1 nmol/L (male) (nmol/L).
Cortisol is the body's primary stress hormone, produced by the adrenal glands in response to physical and psychological stress. It follows a diurnal rhythm — peaking in the early morning (6-8am) to wake you up, then declining through the day. A morning blood test captures your cortisol awakening response, which is the most diagnostically useful measurement.
UK reference range: 166-507 nmol/L (morning) (nmol/L).
Free testosterone is the fraction of total testosterone that circulates unbound to proteins — neither attached to sex hormone-binding globulin (SHBG, which binds roughly 65%) nor to albumin (which loosely binds roughly 33%). Only 1–3% of total testosterone flows freely in the blood, yet this is the only form that can cross cell membranes and activate androgen receptors in muscle, bone, brain, and reproductive tissue. Free testosterone is typically calculated from total testosterone, SHBG, and albumin using the Vermeulen equation — a validated method recommended by the Endocrine Society when direct equilibrium dialysis is unavailable. Because SHBG levels shift with age, body composition, thyroid function, and liver health, two people with identical total testosterone can have very different free testosterone — and very different symptoms.
UK reference range: 0.17–0.66 nmol/L (nmol/L).
Insulin-like Growth Factor 1 is a peptide hormone produced primarily by the liver in response to growth hormone (GH) stimulation. Unlike GH, which is released in pulsatile bursts, IGF-1 circulates at relatively stable levels throughout the day — making it a far more reliable marker of GH axis activity. IGF-1 mediates most of the anabolic effects attributed to growth hormone: muscle protein synthesis, bone density maintenance, tissue repair, and cellular regeneration.
UK reference range: 10–36 nmol/L (age-dependent — declines ~14% per decade after 30) (nmol/L).
Oestradiol (E2) is the primary form of oestrogen in non-pregnant adults, produced mainly by the ovaries in women and in much smaller amounts by the testes and peripheral conversion in men. It regulates the menstrual cycle, bone density, vascular function, mood, cognition, and skin elasticity. In men, oestradiol — converted from testosterone by aromatase — supports brain function, bone health, and libido at the right concentrations, but causes problems when elevated.
UK reference range: Cycle-dependent in women, 41-159 pmol/L in men (pmol/L).
Metabolic
8 markers
HbA1c measures the percentage of haemoglobin that has been glycated (bound to glucose) over the previous 2-3 months. Unlike fasting glucose, which captures a single point in time, HbA1c provides a longer-term picture of blood sugar control. It is the primary marker used to diagnose and monitor type 2 diabetes and pre-diabetes.
UK reference range: 20-42 mmol/mol (below 6.0%) (mmol/mol).
A cholesterol panel measures the fats circulating in your blood — total cholesterol, HDL ('good' cholesterol), LDL ('bad' cholesterol), triglycerides, and increasingly ApoB, the protein that carries LDL particles into artery walls. Together, these markers paint the most accurate picture of your cardiovascular risk and metabolic health available from a blood test.
UK reference range: Total: below 5 mmol/L, LDL: below 3 mmol/L (mmol/L (lipids), g/L (ApoB)).
ALT (alanine aminotransferase) is an enzyme found primarily in liver cells. When the liver is damaged or inflamed, ALT leaks into the bloodstream — making it one of the most sensitive markers of liver health. ALT is more specific to the liver than AST, which is also found in muscle and heart tissue.
UK reference range: 0-41 U/L (U/L).
Apolipoprotein B (ApoB) is the protein embedded in every atherogenic (artery-damaging) lipoprotein particle — including LDL, VLDL, IDL, and Lp(a). Unlike standard LDL cholesterol which measures the cholesterol content inside LDL particles, ApoB counts the actual number of particles. This distinction matters because particle number — not cholesterol content — drives atherosclerosis.
UK reference range: <130 mg/dL (mg/dL).
Lipoprotein(a) is a genetically determined lipoprotein particle structurally similar to LDL but with an additional protein — apolipoprotein(a) — attached. Unlike most cardiovascular risk factors, Lp(a) levels are 80-90% determined by genetics and are largely unresponsive to diet, exercise, or lifestyle changes. It is one of the most underappreciated cardiovascular risk factors, affecting an estimated 1 in 5 people globally.
UK reference range: <75 nmol/L (low risk) (nmol/L).
Homocysteine is an amino acid produced as a byproduct of methionine metabolism. It is not obtained from diet but is generated internally and must be recycled back to methionine (via B12 and folate) or converted to cysteine (via B6). When this recycling pathway is impaired — by nutrient deficiency or genetic variants like MTHFR — homocysteine accumulates in the blood.
UK reference range: <15 µmol/L (µmol/L).
The Omega-3 Index measures the percentage of the omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in your red blood cell membranes, expressed as a proportion of total fatty acids. Because red blood cells have a lifespan of approximately 120 days, this test reflects your average omega-3 status over the previous 3-4 months — far more reliable than a snapshot of plasma levels or a dietary recall questionnaire. The test was first proposed as a standardised risk factor in 2004 and has since been validated in over 40 prospective studies linking it to cardiovascular, cognitive, and inflammatory outcomes.
UK reference range: 4-8% (% of total RBC fatty acids).
Fasting insulin measures the concentration of insulin in your blood after an overnight fast of 8–12 hours. Insulin is a peptide hormone produced by the beta cells of the pancreas in response to rising blood glucose. Its primary role is to shuttle glucose from the bloodstream into cells for energy or storage. When measured in a fasted state — with no incoming glucose from food — insulin reflects your baseline pancreatic output and your tissues' sensitivity to the hormone. A fasting insulin level is one of the earliest markers of metabolic dysfunction, often rising years before fasting glucose or HbA1c moves outside the normal range.
UK reference range: 18–173 pmol/L (3–25 mIU/L) (pmol/L).
Inflammation
2 markers
Ferritin is a protein that stores iron and releases it in a controlled fashion. Blood ferritin levels are the most reliable indicator of total body iron stores. Ferritin also acts as an acute-phase reactant, meaning levels can rise during inflammation, infection, or liver disease — making interpretation context-dependent.
UK reference range: 30-400 ng/mL (ng/mL).
High-sensitivity C-reactive protein is a marker of systemic inflammation produced by the liver. Unlike standard CRP tests used to detect acute infections, hs-CRP measures low-grade chronic inflammation — the kind linked to cardiovascular disease, metabolic dysfunction, and accelerated ageing. It is one of the most validated predictive biomarkers in preventive medicine.
UK reference range: Below 3.0 mg/L (mg/L).
Immune & Inflammation
1 marker
Thyroid
2 markers
Thyroid stimulating hormone is produced by the pituitary gland and signals the thyroid to produce T3 and T4 — the hormones that regulate metabolism, energy production, and body temperature. TSH operates on an inverse relationship: high TSH indicates the thyroid is underperforming (hypothyroidism), while low TSH suggests overactivity (hyperthyroidism).
UK reference range: 0.4-4.0 mIU/L (mIU/L).
Free T3 (triiodothyronine) and Free T4 (thyroxine) are the active thyroid hormones that regulate metabolism, energy production, heart rate, body temperature, and cognitive function. The thyroid gland produces mostly T4, which is converted to the more potent T3 in peripheral tissues — primarily the liver, kidneys, and muscles. 'Free' refers to the unbound, biologically active fraction: only about 0.03% of T4 and 0.3% of T3 circulates freely, with the rest bound to transport proteins (TBG, albumin, transthyretin). It is the free fraction that enters cells and drives metabolic activity.
UK reference range: FT4: 12–22 pmol/L · FT3: 3.1–6.8 pmol/L (pmol/L).
Haematology
1 marker
Full Blood Count
3 markers
Mean corpuscular volume measures the average size of your red blood cells, reported in femtolitres. It is a calculated index from the full blood count and is one of the most diagnostically useful single numbers in blood pathology — it instantly narrows the differential when anaemia is suspected. MCV is also one of the nine biomarkers in the Levine PhenoAge composite, where rising values track biological ageing.
UK reference range: 80-100 fL (fL).
Red cell distribution width measures how much variation there is in the size of your red blood cells — calculated as the coefficient of variation of the red cell volume curve. Where MCV gives the average size, RDW captures the spread. It is one of the most heavily weighted markers in the Levine PhenoAge composite — small absolute changes have large impacts on the bio-age estimate.
UK reference range: 11.5-14.5 % (%).
White blood cell count is the total number of circulating leukocytes — neutrophils, lymphocytes, monocytes, eosinophils, and basophils — measured in ×10⁹ cells per litre of blood. It is the first-line marker of acute infection, but its real value sits at the chronic end: persistent low-grade elevation is one of the most reliable predictors of cardiovascular events, cancer, and mortality. WBC is the ninth biomarker in the Levine PhenoAge composite.
UK reference range: 4.0-11.0 ×10⁹/L (×10⁹/L).
Organ Function
1 marker
Kidney Function
1 marker
Liver & Protein
1 marker
Liver & Bone
1 marker
Common questions
A blood biomarker is a measurable substance in your blood, such as a hormone, vitamin, protein or blood cell count, that reflects how a body system is functioning. Clinicians and researchers use biomarkers to detect deficiencies, track disease risk, and measure the effect of training, diet, and supplementation.
The clinical (NHS) reference range is broad: it covers roughly 95% of the population and is designed to flag disease. The performance-optimal range is tighter, reflecting the level at which research suggests most people feel and function best. You can sit inside the clinical range, be told you are normal, and still be well below optimal.
Yes. Helvy panels measure up to 50+ biomarkers from a finger-prick sample taken at home and posted to a UKAS-accredited UK laboratory. Results arrive in around 5 days, reviewed by a qualified clinician and reported against both clinical and performance-optimal ranges.
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