Mild hyperhomocysteinaemia is an established risk factor for atherosclerosis and vascular disease. However, lowering with vitamin B6 and folate did not improve outcome.
In classic homocystinuria, half the vascular complications are of venous origin. It has been established that high plasma homocysteine levels per se are a risk factor for deep vein thrombosis in the general population. The association was stronger among women than among men, increased with age, and remained significant after excluding other established factors for thrombosis.
In another case control study hyperhomocysteinaemia was found to be a risk factor for thrombosis in people younger than 40 years of age. The difference in the blood levels of homocysteine between controls and patients was particularly evident after a methionine load.
Elevated homocysteine levels may result from low levels of folic acid, vitamin B6, or vitamin B12, some drugs and renal impairment, as well as a number of inherited defects.
It remains unclear whether hyperhomocysteinaemia of different causes entails the same risk of thrombosis. It is also unknown whether vitamin supplementation, which can lower blood levels of homocysteine, will affect the natural history of venous thromboembolism.
The role of Thermolabile Methylene Tetrahydrofolate Reductase (MTHFR) caused by the 677C-->T mutation in thrombophilia is controversial.