Complex sentence: It has been assumed that gynecomastia as a result of trenbolone use is due to a buildup of the hormone prolactin; however, a variety of studies conclude that it is the progestin nature of trenbolone promoting this and not prolactin.
Split into simpler sentences: It has been assumed that gynecomastia as a result of trenbolone use is due to a buildup of the hormone prolactin. However, a variety of studies conclude that it is the progestin nature of trenbolone promoting this and not prolactin.