"Effects of tamoxifen on endometrial estrogen and progesterone receptor concentrations in women with fibrocystic disease of the breast."
Published in Gynecological Endocrinology Magazine (See references).

At some point in time someone read this then came up with a false summation regarding Tamoxifen upregulatation of progesterone receptors. This falsity was then spread about and became a universal truth. Tamoxifen does in fact upregulate pregesterone receptors (PGR) in some individuals as I will explain later, however the Lopez-Comenge trial found that tamoxifen had no effect on progesterone receptors in endometrial tissue at all. That means no upregulation nor any down regulation.

This concept should have been apparent to many from the start as Nolvadex has a strong affinity to receptors in the pectoral region. Hence it's once famous usage tout regarding gynecomastia prevention. As a result of this affinity Nolvadex has very little effect on tissues outside of the pectoral region. I have provided the abstract as a reference point.

Most online medical journal sites have a copy of the entirety of the clinical trials, but for the purposes of this treatise we are concerned with end results thus a abstract proves a suitable alternative since most of you are not willing to pay a subscription fee to view medical trials (though I urge you to start).

I have taken the liberty of emphasizing the key point.

Abstract

Endocrine changes were determined after a 3-week cycle of tamoxifen treatment in 11 regularly cycling women with clinical and radiological evidence of fibrocystic disease of the breast. Blood and endometrial samples were obtained during the luteal phase prior to and at the end of treatment. Tamoxifen treatment (20 mg/day orally for 3 weeks), produced a significant increase in plasma estradiol (p = 0.0018) without simultaneous changes in plasma luteinizing hormone, follicle stimulating hormone, prolactin or progesterone. Tamoxifen treatment significantly reduced endometrial estrogen receptor levels compared to the control cycle (p = 0.0018) while endometrial progesterone receptor levels remained unchanged. Endometrial histological studies showed secretory transformation in both the control cycle and after tamoxifen treatment. The reduction in endometrial estrogen receptor concentrations would suggest a tamoxifen-induced effect or a down-regulatory mechanism to protect target tissues from high estradiol levels. These changes were not associated with alterations in either plasma progesterone or endometrial progesterone receptor concentrations. The tamoxifen-induced changes did not produce any interference in the glandular secretory response of the endometrium. So that is one myth dispelled. Tamoxifen has no effect on receptors other than those found in the pectoral region. So the endometrial progesterone receptor upregulation story is a myth.

Proceeding to the pectoral region of the body we do notice a different story. In the long run Tamoxifen down regulates both progesterone and estrogen receptors. I know this is contrary to popular belief but note I said in the long run meaning 3-4 weeks. A study was done by Dr. N Waseda, Dr. Y Kato, Dr. H Imura, and Dr. M Kurata. The title of the article was:

"Effects of tamoxifen on estrogen and progesterone receptors in human breast cancer."
Published in Cancer Research Magazine (See references)

The results showed that use of tamoxifen during the first two weeks actually did in fact upregulate both estrogen and progesterone receptors. This may seem counter-intuitive to some of you. Your probably thinking. "Wait, if tamoxifen actually upregulates estrogen receptors then why is it used for gynecomastia prevention?" The key element here is that tamoxifen is a SERM. Tamoxifen may upregulate estrogen receptor sites but, it also binds them.

Tamoxifen cannot bind to progesterone receptors though. [B]So in the short run Tamoxifen can in fact aggravate progesterone related gynecomastia for some individuals. Note the study utilized twenty individuals and only 3 out of 5 (60%) had PGR upregulation. In the span of 3-4 weeks those same patients with increased PGR noticed a decrease in PGR levels.

So in summation, in the short run I.E. a span of 1-2 weeks approximately 60% of people will experience progesterone upregulation. In the long run however, EVERYONE will experience PGR downregulation.

So what's my recommendation? Implement Tamoxifen use 4 weeks prior to a 19-nor cycle to down regulate PGR receptors. Of course this whole treatise is a moot point as the use of a prolactin inhibitors makes progesterone gynecomastia a thing of the past.

Once again I have provided the abstract and highlighted key points.

Abstract

Twenty patients with primary breast cancer were treated with tamoxifen (10 mg p.o. twice a day) for 1 to 4 weeks. Before and after the tamoxifen administration, tumor specimens were obtained and assayed for estrogen receptors and progesterone receptors (PGR). Total cytosol estrogen receptor (ERC) and occupied nuclear estrogen receptor (ERN) were measured by hydroxylapatite assay, and unoccupied PGR was measured by the dextran-coated charcoal assay. ERC, ERN, and PGR were detectable in 11, 8, and 6 tumors, respectively, before tamoxifen administration. After tamoxifen treatment, ERC decreased in 10 of 11 ERC-positive tumors. Occupied ERN increased in three of five ERN-positive tumors treated with tamoxifen for a short period (1 to 2 weeks), but they decreased in all of three ERN-positive tumors after longer administration (3 to 4 weeks). PGR increased in three of five ERN-positive tumors after short-term tamoxifen treatment, but they decreased in all of three tumors treated by the drug for a longer period. Increased PGR responses were accompanied by an increase of ERN in two of three ERN-positive tumors. These results suggest that tamoxifen interacts with the estrogen receptor system in human breast cancer tissue and may be estrogenic during short treatment, while longer treatment results in an antiestrogenic response. I of course have several more sources regarding this matter I put the two referenced and two others in my references. I have a total of 31 articles related to this though and I am too lazy to put them all in. If you have questions regarding certain topics I can point you towards an article that may clarify the topic for you.

References
1. Gynecol Endocrinol. 1993 Sep;7(3):185-9. Effects of tamoxifen on endometrial estrogen and progesterone receptor concentrations in women with fibrocystic disease of the breast. Pérez-López FR, Blasco Comenge C. Department of Obstetrics and Gynecology, Hospital Clínico, Zaragoza, Spain.

2. Cancer Res. 1981 May;41(5):1984-8.
   Effects of tamoxifen on estrogen and progesterone receptors in human breast cancer. Waseda N, Kato Y, Imura H, Kurata M.

3. Br J Cancer. 1993 March; 67(3): 606***8211;611. PMCID: PMC1968274
   Effect of tamoxifen on Ki67 labelling index in human breast tumours and its relationship to oestrogen and progesterone receptor status. R. B. Clarke, I. J. Laidlaw, L. J. Jones, A. Howell, and E. Anderson
   Clinical Research Dept., Christie Hospital NHS Trust, Withington, Manchester.

4. Clinical Cancer Research. High Progesterone Receptor Expression Correlates to the Effect of Adjuvant Tamoxifen in Premenopausal Breast Cancer Patients. Maria Stendahl1,2, Lisa Rydén1, Bo Nordenskjöld3, Per Ebbe Jönsson4, Göran Landberg1 and Karin Jirström1