Here is a good explanation I've found for Trenbolone's progestin and prolactin effects.
"Trenbolone is a PgR ligand with mixed agonist/antagonist activity. Perhaps more importantly it also highly suppresses natural progesterone as well as alters its metabolism. Antagonist action at the PgR can be just as much an issue as agonist action. And of course agonist action does not mean that it has the same modulatory effects on ER as progesterone. Nor does it mean that it has the same transcriptional effects as progesterone.
Its not entirely clear whether trenbolone always raises prolactin, in some cases it certainly appears to. It's more consistent effects may be as an allosteric modulator of PRL-R activation and/or expression. However, what can be said with certainty is that prolactin suppression is effective in the treatment of issues that stem from trenbolone, including both libido and gynecomastia."
"Basically just because something is a PgR agonist or a PgR antagonist does not mean that its activity will be the same or opposite of progesterone. Also because these compounds are a bit "slutty", you can be sure that they will be binding to other sites, especially when other hormones are suppressed or when ratios are different. Progesterone is actually a very widely acting hormone and it has numerous active neurosteroid metabolites. Trenbolone metabolites likely have some similarities