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Guide · Gene explainer

UGT2B17 deletion: what it means for testosterone and doping tests

UGT2B17 is the gene for the main enzyme that prepares testosterone to leave the body in urine. Many people are missing it entirely. That makes a large difference to urine tests, and none to who you are.

Quick answer

A UGT2B17 deletion means one or both copies of the gene are missing. People with no copies excrete little or no testosterone glucuronide in urine, which lowers their urine testosterone-to-epitestosterone (T/E) ratio. It is very common in East Asian men and less common in Europeans.

  • Two missing copies: 66.7% of Korean men against 9.3% of Swedish men in one study.
  • After one dose of testosterone, 40% of men with two missing copies never passed the T/E threshold of 4.
  • Consumer DNA chips can't reliably count copies of it; a targeted test is needed.
Gene
UGT2B17, on chromosome 4. Makes UDP-glucuronosyltransferase 2B17.
The variant
A deletion of about 117,000 DNA letters that removes the whole gene. People have 0, 1 or 2 copies.
What it changes
How much testosterone glucuronide leaves the body in urine.
Who has it
Varies by ancestry: very common in East Asian, less common in European populations.
Why it matters
Urine T/E ratio, used in anti-doping tests.
Tested by
Copy-number tests (such as PCR) or sequencing; not reliably by consumer chips.

What the gene does

Before testosterone can be excreted, the body attaches glucuronic acid to it, mostly in the liver. UGT2B17 makes the enzyme that does most of this for testosterone. The gene sits in a region of chromosome 4 where a 117,000-letter stretch is often missing, taking the whole gene with it, so people carry 0, 1 or 2 copies.[1]

How common it is

In a study of healthy men, two missing copies (del/del) were about seven times more common in Korean than in Swedish men:[2]

Korean men (n = 74)66.7%
Swedish men (n = 122)9.3%

Men with two missing copies of UGT2B17. Jakobsson et al. 2006.

Every man with two missing copies in that study had no, or almost no, testosterone glucuronide in his urine, and median urinary testosterone was 16 times higher in the Swedish group, with no difference in creatinine.[2] The pattern of variation around the gene suggests the deletion was favoured by natural selection in East Asia.[1]

Why anti-doping cares

Urine tests for testosterone use the ratio of testosterone to epitestosterone (T/E). The population threshold started at 6 and was lowered by the World Anti-Doping Agency to 4 in 2004.[4] In a 2008 study, 55 men were given a single injection of testosterone. The rise in testosterone glucuronide was on average 20 times larger in men with both copies than in men with none, and 40% of the men with no copies never reached a T/E of 4 on any of the following 15 days.[3]

Since 2014, the steroidal module of the Athlete Biological Passport has compared each athlete with their own earlier samples rather than with a fixed population threshold, and UGT2B17 is recognised as the main source of natural differences in T/E between people.[4]

Testing for it

Because the deletion removes a whole gene rather than changing one letter, it is measured by counting copies, with a targeted PCR copy-number test or sequencing. Consumer genotyping chips aren’t built to count copies, so we don’t recommend reading the deletion from 23andMe or AncestryDNA raw data. The raw DNA checker shows the hormone-gene variants those files can read.

What it can’t tell you

The UGT2B17 deletion doesn’t tell you how you will respond to anabolic steroids or whether they are safe for you.

Questions

What does a UGT2B17 deletion mean?

It means one or both of your copies of chromosome 4 are missing the UGT2B17 gene. The gene makes the main enzyme that attaches glucuronic acid to testosterone so it can leave the body in urine. People with no copies (del/del) excrete little or no testosterone glucuronide in their urine.

How common is the UGT2B17 deletion?

It depends strongly on ancestry. In one study, two missing copies were found in 66.7% of Korean men and 9.3% of Swedish men. It is very common in East Asian populations, where the deletion appears to have been favoured by natural selection.

Does the UGT2B17 deletion affect doping tests?

Yes, it affects urine tests that use the testosterone to epitestosterone (T/E) ratio. In a study where men were given one dose of testosterone, 40% of those with two missing copies never reached the T/E ratio of 4 that was then the threshold. Anti-doping now uses individual profiles over time, the steroidal module of the Athlete Biological Passport, partly for this reason.

Does the deletion change my testosterone level?

The well-documented effect is on how testosterone leaves the body in urine. Blood tests measure testosterone directly, so the deletion's main effect is on urine-based tests rather than on a blood result.

How do I get tested for the UGT2B17 deletion?

With a test that counts copies of the gene, such as a targeted PCR copy-number assay or sequencing. Consumer chips like 23andMe's aren't designed to count copies of a deleted gene, so their raw data can't reliably show it.

Is having the deletion bad for my health?

It is common, and having it isn't a disease. Research has looked at links with how some medicines and hormones are cleared, but we wouldn't draw health conclusions from it on its own.

Sources

  1. Xue Y et al. Adaptive evolution of UGT2B17 copy-number variation. Am J Hum Genet 2008;83:337–46.
  2. Jakobsson J et al. Large differences in testosterone excretion in Korean and Swedish men are strongly associated with a UDP-glucuronosyl transferase 2B17 polymorphism. J Clin Endocrinol Metab 2006;91:687–93.
  3. Schulze JJ et al. Doping test results dependent on genotype of uridine diphospho-glucuronosyl transferase 2B17, the major enzyme for testosterone glucuronidation. J Clin Endocrinol Metab 2008;93:2500–06.
  4. Piper T et al. Current insights into the steroidal module of the Athlete Biological Passport. Int J Sports Med 2021;42:863–78.

For information only, not medical advice. GeneticEdge does not recommend or endorse anabolic steroid use, and no genetic test can tell you whether steroids are safe for you. Talk to a GP or pharmacist about your health. Last reviewed 30 October 2026.