- What it is
- An amino acid in the blood
- Cleared with help from
- Vitamins B12, B6 and folate
- Fasting
- Often 8 to 12 hours
- Routine heart screening
- Not recommended for everyone
Where it comes from
When you digest protein, your body breaks down an amino acid called methionine, and homocysteine is one of the in-between products. Enzymes that depend on vitamin B12, vitamin B6 and folate then convert it into other compounds, so the level in your blood usually stays low.
When one of those vitamins is short, or the conversion pathway runs slowly for another reason, homocysteine builds up.
What can push it up
- Low B12, B6 or folate from diet, poor absorption (more common with age) or certain medicines.
- Reduced kidney function, which also affects how the marker is cleared.
- Underactive thyroid.
- Older age, male sex, menopause and smoking.
- Homocystinuria, a rare inherited disorder that most U.S. newborns are screened for.
The heart-risk question
High homocysteine is linked in studies with damage to artery walls, blood clots, heart attack and stroke. That association made it a popular marker in heart-health panels.
The catch is that treatment trials have been disappointing. Folic acid and other B vitamins reliably lower homocysteine, yet the NIH Office of Dietary Supplements summarizes that they have not reduced overall cardiovascular events or heart attacks; a possible benefit for stroke appears mainly in populations without folic acid fortification. MedlinePlus notes that routine screening of everyone is not recommended because the marker's role in heart disease is uncertain.
Why it still gets ordered
Homocysteine is useful as a supporting test for B12 or folate deficiency, which can cause anemia, nerve symptoms and memory changes. It is a less specific signal than methylmalonic acid for B12, because kidney function and folate also affect it.
In midlife the level tends to drift upward, and women often see an increase after menopause. If yours is high, a clinician will usually look at B12, folate, kidney function and thyroid tests before suggesting any treatment.
Preparing for the test and reading the result
Labs often ask you to fast for 8 to 12 hours, and you should mention any B-vitamin supplements, since they lower the result. Do not stop a prescription to prepare unless your prescriber says so.
A mildly high value is common and is rarely an emergency. The usual next steps are checking B12 and folate status, kidney function and thyroid function, then deciding whether anything needs treating. Repeating the test after a deficiency has been corrected shows whether the underlying cause has been addressed.
Homocysteine: common questions
Should I take B vitamins if my homocysteine is high?
Only after a clinician has looked for the cause. Treating a confirmed B12 or folate deficiency makes sense, but supplements taken just to lower the number have not been shown to prevent heart attacks.
Is homocysteine part of a standard cholesterol test?
No. It is a separate test that has to be ordered specifically.
Does an MTHFR gene variant mean my homocysteine will be high?
Some variants can modestly raise homocysteine, particularly when folate intake is low, but many people with them have normal levels. A blood test tells you more than the gene result alone.
Do I need to fast?
Many labs ask for 8 to 12 hours of fasting. Tell your clinician about any B vitamin supplements you take.
Can medications raise homocysteine?
Some medicines that interfere with B12 or folate absorption or metabolism can. Do not stop any prescription without talking to the prescriber.
Is a high level dangerous on its own?
A high result is a reason to investigate, not a diagnosis. It is interpreted alongside your other risk factors and labs.
Sources
We cite government agencies, professional societies and peer-reviewed research. See our editorial policy.
- Homocysteine testMedlinePlus (U.S. National Library of Medicine)
- Folate: Fact Sheet for Health ProfessionalsNIH Office of Dietary Supplements
- Vitamin B12: Fact Sheet for Health ProfessionalsNIH Office of Dietary Supplements