Yep and that build up of progesterone leads to prolactin my friend.
Quoting wiki which is shit, but it backs it up in this case. Progesterone is the culprit, and prolactin the end result.
“However, due to trenbolone's potent
progestogenic activity,
gynecomastia, which is characterized by development and swelling of breast tissue,
[28]may still be possible.[
citation needed] Stimulation of estrogenic mechanisms are enforced by progestogenic activity[
citation needed] as trenbolone acetate and its compounds bind with high affinity to the
progesterone receptor.
[11][20] It has been assumed that gynecomastia as a result of trenbolone use is due to a buildup of the hormone
prolactin;
[28] however, a variety of studies conclude that it is the progestogenic activity of trenbolone promoting this and not prolactin.[
citation needed] Trenbolone also has a negative impact on blood pressure, but it does not appear to negatively affect most healthy adult men in this way.”
en.m.wikipedia.org