Context: It is generally assumed that delayed endometrial development observed in luteal phase deficiency (LPD) is the result of abnormally low Progesterone (P) levels. This hypothesis has never been tested by direct experiment.

Objective: To evaluate the effects of P concentrations on human endometrium

Design: Randomized trial

Setting: Academic medical center

Subjects: 29 healthy, ovulatory 18–35 year old women

Intervention: Endometrial samples were obtained from women in natural cycles and two groups of experimentally-modeled cycles. Women undergoing modeled cycles were treated with GnRH agonist and a fixed physiologic dose of transdermal estradiol, followed by randomization to 10 or 40 mg daily IM P administration to achieve either normal circulating luteal P or four-fold lower P concentrations, the latter representing an experimental model of LPD.

Main Outcome Measured: Tissue specimens, obtained after ten days of P exposure, were analyzed by histologic dating, immunohistochemistry, immunoblot, and real-time reverse-transcriptase polymerase chain reaction (qRT-PCR).

Results: Histologic dating of endometrium, immunohistochemistry for endometrial integrins and qRT-PCR analysis for 9 putative functional markers showed no differences between the three groups. Preliminary data from western analysis suggest that some proteins may be affected by low serum P concentrations.

Conclusions: Histologic endometrial dating does not reflect circulating P concentrations and cannot serve as a reliable bioassay of the quality of luteal function. Assessment of selected functional markers by either immunohistochemistry or qRT-PCR is similarly insensitive to decreased circulating P. Preliminary evidence suggests that abnormally low luteal phase serum P concentrations may have important functional consequences not otherwise detected.