Epitestosteron, eller isotestosteron, også kjent som 17a-testosteron eller som androst-4-en-17a-ol-3-on, er et endogent steroid og en epimer av androgen kjønnshormonet testosteron. Det er en svak konkurrerende antagonist av androgenreseptoren (AR) og en potent 5a-reduktasehemmer.
Strukturelt sett, epitestosteron skiller seg fra testosteron bare i konfigurasjonen ved det hydroksybærende karbonet, C17. Epitestosterone is believed to form in a similar way to testosterone; en 1993 study found that around 50% of epitestosterone production in human males can be ascribed to the testis, although the exact pathway of its formation is still the subject of research. It has been shown to accumulate in mammary cyst fluid and in the prostate.Epitestosterone levels are typically highest in young males; imidlertid, by adulthood, most healthy males exhibit a testosterone to epitestosterone ratio (T/E ratio) of about 1:1
