Horse Side Vet Guide ®

Equine Health Resource

Testosterone Level in Serum



Testosterone is the hormone responsible for stallion sexual behavior. It is also responsible for stallion-like appearance, the so called secondary sex characteristics of heavy muscling in the neck and jowels.

Testosterone testing is usually done in geldings that are showing stallion-like behavior. It involves taking a sample of blood and submitting it to a laboratory. A positive test indicates that the horse has testicular tissue and may warrant surgical exploration for a retained testicle. While a retained testicle does not usually produce viable sperm, it does continue to produce testosterone.

(It is not considered acceptable veterinary practice to only remove one testicle during a castration, but unfortunately it does occur. Vets typically examine a stallion prior to castration to ensure that both testicles are present and down.)

Related Diagnostics


A low value of testosterone usually indicates that the horse is a gelding. A high value indicates the presence of testicular tissue.


There are cases in which the testosterone result is on the high side for a gelding and on the low side for a stallion. In those cases, modified tests of other hormones (estrone sulfate) or what is called an HCG stimulation test are recommended to provide greater certainty.


  • Does this test confirm that there is actually testicular tissue present in my horse?
  • Are other tests necessary to confirm?
  • Author: Doug Thal DVM Dipl. ABVP


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