Homocysteine is a sulphur-containing amino acid produced in the body as an intermediate product of methionine metabolism. It is normally present in the blood in small amounts and is rapidly converted to other compounds, a process requiring B vitamins (folate, B6, and B12) as co-factors. When these vitamins are deficient, or in people with certain genetic variants, homocysteine accumulates in the blood.
Elevated homocysteine levels (hyperhomocysteinaemia) have been associated with an increased risk of cardiovascular disease, including coronary artery disease, stroke, and venous thrombosis. Proposed mechanisms include endothelial dysfunction (damage to the inner lining of blood vessels), acceleration of atherosclerosis, and pro-thrombotic effects. However, clinical trials investigating whether lowering homocysteine with B vitamin supplementation reduces cardiovascular events have largely been negative, suggesting that homocysteine may be a marker of cardiovascular risk rather than a direct cause.
Causes of elevated homocysteine include deficiency of folate, vitamin B12, or vitamin B6 (the most common cause), genetic variants affecting folate metabolism (particularly MTHFR polymorphisms), renal impairment, hypothyroidism, and certain medications. Correction of nutritional deficiencies is the primary approach where relevant.
Testing for homocysteine is not part of routine cardiovascular risk assessment in the UK. It may be measured in selected patients with premature cardiovascular disease, recurrent thrombosis, or when an inherited metabolic disorder is suspected.
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