[anyover88]

Actually they use a gel for vaginal and rectal administration and transdermal is also administered through oil dosed with USP progesterone. Most of what you quoted is about transdermal, which is most typically administered with a cream, the oil type is pretty rare, but you did quote a little about vaginal/rectal as well, which is most definitely never administered through a cream.

This page actually touches upon that one

http://www.progesteronetherapy.com/progesterone-test.html

A second study by Rachel Miles, MD, and associates (17) demonstrated that serum testing of progesterone is not reflective of tissue uptake when progesterone is used as a vagina suppository. In this study, progesterone levels in serum and tissue were compared after intramuscular and vaginal delivery of progesterone. After treatment with either intramuscular injection of progesterone or vaginal progesterone suppositories, serum and uterine biopsies were taken to measure blood and tissue uptake of progesterone. Serum levels of progesterone (measured by RIA) were three times higher with intramuscular delivery of progesterone than with vaginal delivery. In striking contrast, tissue uptake in the uterine biopsies was 10 times higher with vaginal delivery of progesterone than with intramuscular injections. For lack of a better understanding, these authors attributed the tissue differences to a "first pass effect," a term used to describe local diffusion of progesterone from the vagina to the uterus without significant systemic delivery to other tissues. (18) However, these authors could not prove or disprove what they described as a "first pass effect" because they did not biopsy other tissues.

Vaginal delivery of progesterone into the body is, in essence, through the epithelial layer of skin and does not differ in this regard from other forms of topical progesterone delivery. Therefore, a more likely explanation for the discrepant serum/tissue results is that when progesterone is delivered vaginally it is rapidly delivered to all tissues throughout the body. The manufacturers of Crinone, a vaginal suppository progesterone gel, make the same claims of a "first pass effect," yet also claim that women have significant improvement in well being, indicating that progesterone also finds its way from the vagina to the brain as well as the uterus. (19)

Edit: here is the original source with all citations at the end http://faculty.ksu.edu.sa/drzeinab/Publications/townsend%20letter[1].pdf

and here is a more up to date article by the same author on the same subject
http://www.townsendletter.com/Jan2014/troubleproges0114.html