Estradici and Progesterone Receptor Levels in Human Breast Adenocarcinoma in Relation to Plasma Estrogen and Progesterone Levels1

Plasma estrogen and progesterone levels were deter mined in 77 premenopausal and 137 menopausa! women at the same time that estradiol receptor (ER) and proges terone receptor (PGR) assays were carried out on their breast cancers. The frequency of ER and PGR is approxi mately the same in premenopausal and postmenopausal women, but the ER content is much higher in postmeno pausal women. Although this is usually ascribed to the occupancy of receptors by endogenous estrogen in pre menopausal women, our observations suggest that this is unlikely. The higher ER content in postmenopausal women is probably due to the fact that the cyclic proges terone increase in premenopausal women limits estrogen stimulation of ER synthesis. Our data suggest that the circulating levels of estrogen in postmenopausal women are sufficient to stimulate ER and PGR when ER is functional. In premenopausal women, on the other hand, high levels of circulating progesterone may inhibit PGR, and the absence of PGR in the breast cancers of premenopausal women should be interpreted warily if the plasma level of progesterone is unknown.