Since I just gave somebody else a hard time about a very similar claim, I have to ask if there's any evidence this would be noticeable with sane doses of progesterone? If progesterone's opposition to estradiol works via the progesterone receptor—and ER downregulation—then the whole process would seem to be pretty slow, thus minimizing any changes seen in serum estradiol.
very great point Cat, maybe there is another missing piece of the puzzle and/or this would only be seen in some very estrogen dominant people where there is serious metabolic concerns and the liver to show for it. Maybe it’s not likely to see any serum increase for the vast majority of people. I know this did not happen to me when adding progesterone.
unless it is quick due to some other competitive mechanism preventing E from binding and the body adjusts before bloodwork is done due to most people waiting several weeks before getting bloodwork on any given protocol addition/change.