Homocysteine
Amino acid intermediate as vascular and cognition marker
Also known as
Definition
Homocysteine is a sulfur-containing amino acid derived from methionine metabolism, broken down by folate, vitamin B6, and vitamin B12. Elevated levels are an independent risk factor for heart disease, dementia, and stroke. Elevated levels (hyperhomocysteinemia) occur more often with a vegan diet and with MTHFR polymorphisms.
| Parameter | Value |
|---|---|
| Unit | µmol/l |
| Reference Range | < 15 µmol/l (general) |
Agree on your personal target range with your doctor.
↓ What a low value means
Low homocysteine values are considered clinically not relevant. Your doctor interprets the result.
↑ What a high value means
Values above 15 µmol/l are referred to as hyperhomocysteinemia; elevated levels are considered an independent risk factor for atherosclerosis, dementia, and stroke. The literature also grades by level, as for example a stroke study does (Ashjazadeh et al., Iranian Journal of Medical Sciences 2013): moderate 15–30, intermediate above 30 to 100, and severe above 100 µmol/l. This grading describes how high the measured value is; it is not a diagnosis. Very high values are rare and often genetic (CBS or MTHFR defects). Causes: folate, B12, and B6 deficiency (common in vegans), MTHFR C677T polymorphism, kidney insufficiency, hypothyroidism, certain medications (methotrexate, metformin). Your doctor interprets the result.
✓ What can influence this value
The homocysteine value is influenced by the supply of folate, vitamin B12, and vitamin B6, which take part as cofactors in its breakdown. Genetic variants such as the MTHFR polymorphism, kidney function, age, alcohol consumption, smoking, and an underactive thyroid also act on the value. Medications such as metformin (inhibits B12 absorption), methotrexate, or antiepileptic drugs can raise the value. A protein-rich meal before the blood draw can raise it briefly. The assessment is made by the treating doctor.
When to test
The measurement is preferably done fasting, because meals raise homocysteine temporarily. Possible reasons for testing are a vegan diet, an MTHFR polymorphism, a previous stroke, a risk of dementia, or treatment with metformin or methotrexate. Your doctor decides how often it is measured.
Frequently asked questions
What is the MTHFR polymorphism and how does it relate to homocysteine? +
MTHFR (methylenetetrahydrofolate reductase) is an enzyme that converts folic acid to its active form (methylfolate). The common C677T variant reduces enzyme activity by 30–70%, making homocysteine detoxification more difficult.
Does B-vitamin supplementation lower heart attack risk with high homocysteine? +
B vitamins reliably lower homocysteine, but direct evidence of cardiovascular risk reduction through supplementation (without deficiency) is inconsistent in RCTs.
Do I need to fast for homocysteine testing? +
Usually yes: a protein-rich meal can raise homocysteine briefly, so the measurement is preferably done fasting. If the sample was not taken fasting, this is taken into account when interpreting the result. Your practice sets the exact instructions.
Homocysteine
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.