Homocysteine
An amino acid tied to B-vitamin status and cardiovascular risk.
Homocysteine is an amino acid produced as an intermediate when the body processes methionine. It is normally cleared quickly along two routes, both of which depend on B vitamins: folate and B12 for one, vitamin B6 for the other.
Raised levels therefore usually indicate a shortfall in one of those vitamins, or impaired kidney function, or a genetic variation in the enzymes involved. That makes it a reasonable pointer towards B vitamin status when there is a clinical reason to look.
Its fame comes from a hypothesis that has not held up. Observational studies consistently associate high homocysteine with cardiovascular disease and dementia, and the obvious inference was that lowering it would help. Large randomised trials of B vitamin supplementation lowered homocysteine reliably and did not reduce cardiovascular events.
That gap is one of the cleanest teaching examples in cardiology: an association can be real, reproducible and biologically plausible, and still not be causal. Homocysteine now looks more like a marker of something else than a mechanism to target.
The MTHFR variants that raise it mildly are common, heavily marketed by direct to consumer testing, and clinically far less significant than that marketing implies in people who are not deficient.
It still has legitimate clinical uses, particularly in investigating unexplained B12 or folate deficiency, in rare inherited metabolic disorders, and occasionally in unexplained clotting.
In practice, it is not a marker worth adding to a routine panel out of curiosity, and a mildly raised result is a reason to check B12 and folate rather than to start a supplement regime on the assumption that lowering the number lowers the risk.
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
Women
- Below typical
- under 2 µmol/L
- Typical range
- 2 to 11.2 µmol/L
- Above typical
- 11.2 µmol/L and over
The result is sensitive to how quickly the sample reached the laboratory: a delay before separation raises it on its own, independently of anything about you. Reference intervals differ between laboratories and assays. Where your own report prints a range, the dashboard draws that one instead of this.
Reference interval from North Bristol NHS Trust, Severn Pathology. Read against that page on 2026-08-09. Your own laboratory may print a slightly different range.
Men
- Below typical
- under 2 µmol/L
- Typical range
- 2 to 14.2 µmol/L
- Above typical
- 14.2 µmol/L and over
The result is sensitive to how quickly the sample reached the laboratory: a delay before separation raises it on its own, independently of anything about you. Reference intervals differ between laboratories and assays. Where your own report prints a range, the dashboard draws that one instead of this.
Reference interval from North Bristol NHS Trust, Severn Pathology. Read against that page on 2026-08-09. Your own laboratory may print a slightly different range.
On your own blood test?
Homocysteine is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, your own laboratory’s printed range is used in place of the one above wherever your report carries one, and your results are explained together in one place.