What is a normal homocysteine level?

Clinical references give a typical range of 5 to 15 µmol/L and define hyperhomocysteinemia as above 15 µmol/L [1]. ARUP reports an adult reference interval of 0 to 15 µmol/L [6]. Labcorp uses age bands, shown in the chart [2].

Lab intervals describe where most tested people fall, not what is optimal. They come from local populations, and populations eating folic-acid-fortified food run lower: after US fortification, mean homocysteine fell from 10.1 to 9.4 µmol/L [7].

Always compare your result with the range printed on your own report, and with your own earlier results measured the same way.

The most specific proposal comes from Smith and Refsum, who reviewed more than 100 conditions linked to homocysteine and concluded that adult values of 10 µmol/L or below are probably safe and 11 µmol/L or above may justify intervention [3]. The dementia consensus describes values above 11 µmol/L as moderately raised in older people [8].

Trial data support thresholds in this region for brain outcomes. In VITACOG, the B-vitamin benefit on brain atrophy was confined to people above the median of 11 µmol/L [9] and reached 53% above 13 µmol/L [10].

For heart attacks, there is no evidence that pushing homocysteine lower helps: trials that cut it by 25% did not reduce heart attacks [11]. A "good homocysteine number" therefore depends on the question. For brain health in older adults, under about 10 to 11 is the researchers' target. For cardiovascular risk, other markers, such as ApoB, carry more weight.

The Hordaland Homocysteine Study measured 16,176 Norwegian adults aged 40 to 67 [4]:

Age and kidney function, measured as creatinine, are both independent determinants of homocysteine [12]. Lab ranges follow the same pattern. Labcorp's upper limits, the same for men and women, run from 9.0 µmol/L at 2 to 12 years to 14.5 at 18 to 60, 17.2 at 61 to 70, 19.2 at 71 to 80 and 21.3 over 80 [2].

Women. Folic acid lowers homocysteine more in women than in men [13]. Most labs do not publish a separate female range; ask your lab whether yours does.

Men. Men had higher levels than women at every age in the Hordaland data, and smoking widened the gap [4].

Reference intervals start at 0 µmol/L at Labcorp and ARUP [2][6], so a low result is not flagged as abnormal. The typical adult range starts at about 5 µmol/L [1].

The few studies of low values are observational:

• In 2,655 Korean adults over 60, people at 8.9 µmol/L or below had 4 to 5 times the dementia risk of those at 9.0 to 10.5, similar to people at 10.6 or above, and 41.2% of the low group took vitamin supplements [5].
• In a retrospective review of 37,442 clinic patients, very low homocysteine was uncommon but linked to idiopathic peripheral neuropathy [14].

These studies cannot show that low homocysteine causes harm. They do suggest that pushing values ever lower with high-dose supplements has no proven benefit. The Hordaland data found low homocysteine associated with better physical and mental health overall [12].

Homocysteine is not a waste product. It is the step between methionine and cysteine, which the body uses for glutathione and taurine [1]. In theory, very low homocysteine could reflect a pathway running in an unusual way.

In practice, data are limited to the observational studies above [5][14]. No trial has tested raising homocysteine. If your result is low and you take high-dose B vitamins or methyl donors, the simplest question for your clinician is whether you still need that dose. Searches for "how to increase homocysteine naturally" rarely have a medical answer beyond reviewing supplements.

Homocysteine has a molar mass of about 135 g/mol, so 1 mg/L equals about 7.4 µmol/L. Published papers show the pairing: 3 µmol/L is 0.41 mg/L [15] and 2.4 µmol/L is 0.3 mg/L [16].

• 10 µmol/L is about 1.35 mg/L
• 15 µmol/L is about 2.0 mg/L
• 30 µmol/L is about 4.1 mg/L

Some reports write µmol/L as umol/L or micromol/L; they are the same unit. See what high homocysteine means if your value is above the range.