Quinestrol is a synthetic estrogen that is effective in hormone replacement therapy. It is a 3-cyclopentyl ether of ethynyl estradiol. After gastrointestinal absorption, it is stored in adipose tissue, where it is slowly released and metabolized in the liver to its active form, ethinyl estradiol. Quinestrol has found limited use in suppressing lactation in postpartum women and, in combination with synthetic progestogens, as contraceptive therapy, although additional studies are needed for both applications. Estrogens diffuse into their target cells and interact with a protein receptor (the estrogen receptor). Estrogen interacts with a target cell receptor. When the estrogen receptor has bound its ligand it can enter the nucleus of the target cell, and regulate gene transcription which leads to formation of messenger RNA. The mRNA interacts with ribosomes to produce specific proteins that express the effect of estradiol upon the target cell. Estrogens increase the hepatic synthesis of sex hormone binding globulin (SHBG), thyroid-binding globulin (TBG), and other serum proteins and suppress follicle-stimulating hormone (FSH) from the anterior pituitary. Target cells include the female reproductive tract, the mammary gland, the hypothalamus, and the pituitary. Estrogens increase the hepatic synthesis of sex hormone binding globulin (SHBG), thyroid-binding globulin (TBG), and other serum proteins and suppress follicle-stimulating hormone (FSH) from the anterior pituitary. The combination of an estrogen with a progestin suppresses the hypothalamic-pituitary system, decreasing the secretion of gonadotropin-releasing hormone (GnRH).