What does high homocysteine mean?

Updated October 31, 2026 · 32 sources · Written by the AumniVIDA team

Units

Short answer

High homocysteine, above about 15 µmol/L, usually means a folate or B12 shortfall, reduced kidney function, hypothyroidism, a drug effect or the MTHFR TT genotype with low folate. Values above 30 point to significant deficiency or kidney disease, and above 100 to inherited disorders. It predicts vascular and brain risk but rarely causes symptoms itself.

Homocysteine categories in µmol/L

10 or below: probably safe (Smith and Refsum): 0 to 10
10 to 15: within lab range, above expert target: 10 to 15
15 to 30: moderate: 15 to 30
30 to 100: intermediate (over 100 severe): 30 to 40

10 µmol/L: Probably safe at or below
11 µmol/L: Dementia risk threshold in consensus
13 µmol/L: VITACOG benefit above
14.5 µmol/L: Labcorp upper limit, age 18 to 60
15 µmol/L: ARUP upper limit
30 µmol/L: Intermediate starts

Most adults sit between 5 and 15 µmol/L. Moderate, intermediate and severe hyperhomocysteinemia start at about 15, 30 and 100 µmol/L, and the cutoffs that experts and labs use for action differ by several units.

Key takeaways

1
Above 15 µmol/L is hyperhomocysteinemia; moderate to 30, intermediate to 100, severe beyond [1][2].
2
In 165 patients with cardiovascular events and raised homocysteine, mostly above 30 µmol/L, 55% had B12 or folate deficiency, 15% severe kidney failure and 11% inherited disorders [1].
3
High homocysteine itself causes no specific symptoms; symptoms come from the cause, such as B12 deficiency [3].
4
It predicts heart disease and stroke [4], but lowering it has not reduced heart attacks [5].

What does a high homocysteine level mean?

It means homocysteine is being made faster than folate, B12 and B6 clear it, or that the kidneys clear it slowly. How high it is gives a clue to the cause.

Most adults sit between 5 and 15 µmol/L [2]. Above that, the size of the rise is the first clue:

15 to 30 µmol/L, moderate. Common. Usually mild folate or B12 shortfall, lower kidney function, older age, smoking, heavy coffee, hypothyroidism or the MTHFR TT genotype with low folate [6][7].
30 to 100 µmol/L, intermediate. Usually significant B12 or folate deficiency or kidney failure [1].
Over 100 µmol/L, severe. Points to inherited disorders such as cystathionine beta-synthase deficiency, or severe B12 deficiency [2][1].

Before drawing conclusions, check the sample conditions. A non-fasting draw after a large protein meal can read about 13% higher [8], and blood left unspun at room temperature rises about 0.7 µmol/L per hour [9]. Lab ranges also climb with age; Labcorp's upper limit is 14.5 µmol/L at 18 to 60 years and 21.3 µmol/L over 80 [10].

Homocysteine categories in µmol/L

10 or below: probably safe (Smith and Refsum): 0 to 10
10 to 15: within lab range, above expert target: 10 to 15
15 to 30: moderate: 15 to 30
30 to 100: intermediate (over 100 severe): 30 to 40

10 µmol/L: Probably safe at or below
11 µmol/L: Dementia risk threshold in consensus
13 µmol/L: VITACOG benefit above
14.5 µmol/L: Labcorp upper limit, age 18 to 60
15 µmol/L: ARUP upper limit
30 µmol/L: Intermediate starts

What causes high homocysteine?

Low B12 or folate, reduced kidney function, hypothyroidism, metformin, the MTHFR TT genotype with low folate, smoking, heavy coffee and age. Rare inherited disorders cause the highest values.

The table lists the main causes with the size of effect reported in studies.

A few causes deserve detail:

B12. Long-term metformin lowered B12 by 19%, and patients who became deficient averaged 23.7 µmol/L [11]. Vegans and vegetarians without supplements run higher homocysteine than omnivores [12].
Folate. Folic acid fortification cut the share of US adults above 13 µmol/L from 18.7% to 9.8% [13]. Where food is fortified, folate deficiency is a less common cause.
Kidneys. Higher creatinine tracks higher homocysteine [6]. Kidney function is worth checking alongside the result.
Thyroid. Overt hypothyroidism raises homocysteine; even subclinical hypothyroidism adds about 1.16 µmol/L [14]. The TSH hub covers thyroid testing.
Proton pump inhibitors are often listed as a cause, but in 25,953 adults the difference was only 12.0 versus 11.6 µmol/L [15].

Causes of high homocysteine and how big their effect is.
Cause	Typical effect	Source
Vitamin B12 deficiency on metformin	Mean 23.7 µmol/L in deficient patients vs 14.9 with normal B12	de Jager 2010
Low folate before fortification	18.7% above 13 µmol/L, falling to 9.8% after	Jacques 1999
MTHFR TT in low-folate regions	About 3.1 µmol/L higher than CC	Holmes 2011
MTHFR TT in fortified regions	About 0.1 µmol/L higher than CC	Holmes 2011
Heavy filtered coffee (1 L a day)	1.5 µmol/L (18%) higher	Urgert 2000
Heavy smoking, older age, male sex	Up to 4.8 µmol/L higher than young non-smoking women	Nygård 1995
Subclinical hypothyroidism	About 1.16 µmol/L higher	Zhang 2020
Proton pump inhibitors	12.0 vs 11.6 µmol/L, not clinically relevant	Lerman 2022
Severe kidney failure	15% of a hospital series, mostly above 30 µmol/L	Levy 2021
Inherited one-carbon disorders	11% of the same series; often over 100 µmol/L	Levy 2021; StatPearls

What are the symptoms of high homocysteine?

Moderately high homocysteine has no symptoms of its own. Symptoms, when present, come from the cause: B12 deficiency, kidney disease, hypothyroidism or, rarely, an inherited disorder.

Homocysteine is found on a blood test, not felt. When people with high results have symptoms, these usually reflect the underlying problem:

B12 deficiency can cause megaloblastic anaemia and neurological problems such as numbness, tingling, balance trouble and memory change; left untreated it can be severe [3]. Treatment should not be delayed when symptoms are strong, even if tests are borderline [16].
Intermediate and severe hyperhomocysteinemia has been found in patients admitted with blood clots and other cardiovascular events, and in one series it was the only vascular risk factor in about 9% [1].
Classical homocystinuria, the rare inherited form, damages blood vessels and bones from a young age [2][17][18].
Hypothyroidism brings tiredness, cold intolerance and weight gain; see the TSH hub.

Lists online that link high homocysteine to fatigue, brain fog or anxiety usually describe B12 deficiency or another cause, not homocysteine itself.

Is high homocysteine dangerous for the heart, brain and bones?

It predicts higher risk of heart disease, stroke, dementia and fractures. Trials show lowering it does not prevent heart attacks, may modestly lower stroke, and slows brain atrophy in older adults with high levels.

Heart and stroke. A 25% lower homocysteine was associated with 11% less heart disease and 19% less stroke in cohorts [4]. B-vitamin trials lowered homocysteine by about 25% without changing heart attacks (RR 1.02), with a small stroke reduction (RR 0.90) [5]. Genetic evidence points the same way: little or no coronary effect [19], some stroke effect in low-folate populations [7][20]. Risk is better judged with markers that trials have validated, such as ApoB, LDL cholesterol and lipoprotein(a).

Brain. Homocysteine above 14 µmol/L nearly doubled Alzheimer risk in Framingham [21], and consensus experts call it a modifiable risk factor [22]. B vitamins slowed brain atrophy in people with mild cognitive impairment and high homocysteine [23][24], but dementia prevention is not proven.

Bones. The top homocysteine quartile carried about 1.9 times the fracture risk in older adults [18][25]. Trials of B vitamins for fracture prevention are fewer and are not reviewed here.

Can stress raise homocysteine levels?

There is little human evidence that psychological stress itself raises homocysteine. Habits that often travel with stress, such as smoking, heavy coffee and a poor diet, do.

We found no large human study showing that psychological stress raises homocysteine. Most studies on stress and homocysteine are in animals or cells, which cannot tell you what happens in a person's blood test.

What does move homocysteine is well documented:

Smoking: levels climb with each cigarette per day [26].
Heavy coffee: a litre a day raised it by 1.5 µmol/L [27].
Diet low in folate: a fruit, vegetable and dairy rich diet lowered it modestly [28].
Inactivity: physical activity tracked with lower levels [26].

If a stressful period came with more coffee, more cigarettes and less cooking, those changes are the likelier link.

What usually happens after a high homocysteine result?

Clinicians usually repeat it under proper conditions and look for the cause: B12, MMA, folate, kidney function and thyroid. Genetic MTHFR testing is not recommended routinely.

The next steps are a clinician's call, but a typical work-up follows the causes:

Repeat the test properly: fasting, seated, processed on time [29][30].
Check B12 status, ideally with methylmalonic acid, which is more specific for B12 than homocysteine [16].
Check folate. Serum folate is the first-line folate test [16].
Check kidney and thyroid function [6][14].
Review medications such as metformin [11].
Consider inherited disorders if values are very high or start in childhood [1].

The American College of Medical Genetics advises against routine MTHFR testing [31], because the homocysteine level itself already shows whether the pathway is working. Treatment of an identified deficiency is straightforward; see how to lower homocysteine.

More about Homocysteine

Protocols that act on Homocysteine

Common questions