Exogenous estrogens are metabolized in the same manner as endogenous estrogens. Circulating estrogens exist in a dynamic equilibrium of metabolic interconversions. These transformations take place mainly in the liver. Estradiol is converted reversibly to estrone, and both can be converted to estriol, which is the major urinary metabolite. Estrogens also undergo enterohepatic recirculation via sulfate and glucuronide conjugation in the liver, biliary secretion of conjugates into the intestine, and hydrolysis in the gut followed by reabsorption. In postmenopausal women, a significant proportion of the circulating estrogens exist as sulfate conjugates, especially estrone sulfate, which serves as a circulating reservoir for the formation of more active estrogens.
In the liver estradiol is readily oxidised to estrone and both estradiol and estrone are converted by hydration to estriol. Metabolites of estrogens are mainly excreted in the urine as conjugates of glucuronic and sulphuric acid.
The 17beta-hydroxy steroid dehydrogenase transforms estrone to estradiol reversibly. This enzyme occurred in all tissues of all species examined and is linked to either the cytosolic or microsomal subcellular compartment. In human liver, a NAD-linked 17beta-hydroxy steroid 3-hydrogenase occurs in cytosol and in microsomes, and a further NADP-linked enzyme has been found in cytosol. Hence, estrone and estradiol are largely biologically equivalent; they are also metabolized via the same pathways.
The steroidal estrogens are metabolized principally in the liver, although the kidneys, gonads, and muscle tissues may be involved to some extent. The steroids and their metabolites are conjugated at the hydroxyl group of the C 3 position with sulfuric or glucuronic acid; these conjugates may undergo further metabolic change. Conjugation increases water solubility and facilitates excretion in urine. Large amounts of free estrogens are also distributed into the bile, reabsorbed from the GI tract, and recirculated through the liver where further degradation occurs. /Estrogen General Statement/
Estrogens enter the cells of responsive tissues (e.g. female organs, breasts, hypothalamus, pituitary) where they interact with estrogen receptors. Hormone-bound estrogen receptors dimerize, translocate to the nucleus of cells and bind to estrogen response elements (ERE) of genes. Binding to ERE alters the transcription rate of affected genes. 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) release from the anterior pituitary.
The natural estrogens are generally quickly and well absorbed from the gastrointestinal tract, there being little difference between estrone, estradiol and estriol. Estrogens are inactivated in the liver. A proportion of absorbed estrogen is excreted in the bile and then reabsorbed from the intestine.
The distribution of exogenous estrogens is similar to that of endogenous estrogens. Estrogens are widely distributed in the body and are generally found in higher concentrations in the sex hormone target organs. Estrogens circulate in the blood largely bound to sex hormone binding globulin (SHBG) and albumin.
Following administration of Hormonin, physiological estrogen concentrations are achieved at different rates. The maximum plasma levels and the time to reach the peak in the plasma level varied between subjects and was; estrone 750-2116 pmol/litre, 0.5-6.0 hours; estradiol 246-813 pmol/litre, 1-8 hours; estriol 173-241 pmol/litre, 5-12 hours. Following steady state conditions after cessation of Hormonin therapy, estrogen levels remained in the pre-menopausal range for approximately 48 hours.
Estrogens are available for oral, parenteral, transdermal, or topical administration ... absorption is generally good with the appropriate preparation. /Estrogens/