 Testosterone isobutyrate is a prodrug of testosterone, and hence is an agonist of the androgen receptor, the biological target of endogenous androgens like testosterone and dihydrotestosterone.It produces both androgenic and anabolic effects, as well as weak estrogenic effects due to metabolism of testosterone into estradiol.In contrast to most other testosterone esters, which are used as amorphous oil solutions, testosterone isobutyrate is provided in the form of a microcrystalline aqueous suspension.