Gonadotropins, such as luteinizing hormone and follicle-stimulating hormone, are pituitary hormones. These hormones consist of a common α subunit and a unique β subunit. Postmenopausal women's urine is a valuable source of gonadotropins because their serum gonadotropin levels are high due to the absence of negative feedback by ovarian steroids. The hormones are filtered and excreted as intact molecules in urine. Another gonadotropin is human chorionic gonadotropin (hCG), a placental hormone that has the same common α subunit and an hCG-specific β subunit. hCG is a stable hormone, and blood hCG levels double every 2 days during the first trimester of pregnancy. Consequently, urinary levels of hCG also increase rapidly. Pregnancy tests detect the hCG-specific β subunit in urine as part of the intact hCG heterodimer using immunological methods.

Steroid hormones are produced through a series of enzymatic modifications of cholesterol. Their core structure is a cyclopentanoperhydrophenanthrene ring or a derivative thereof. The enzymatic modifications of cholesterol fall into three general types: hydroxylation, dehydrogenation/reduction, and lyase reactions. These modifications aim to generate a cholesterol derivative that is distinctive enough to be recognized by a specific receptor. Progestins bind to the progesterone receptor, mineralocorticoids bind to the mineralocorticoid receptor, glucocorticoids bind to the glucocorticoid receptor, androgens bind to the androgen receptor, estrogens bind to the estrogen receptor, and the active vitamin D metabolite binds to the vitamin D receptor. The complexity of steroid hormone action is heightened by the presence of multiple forms of each receptor. Moreover, there is some degree of overlap between steroid hormones and the receptors they bind to. For instance, glucocorticoids have a high affinity for the mineralocorticoid receptor, and progestins, glucocorticoids, and androgens can all interact to some extent with the progesterone, glucocorticoid, and androgen receptors. As discussed later, steroid hormones are hydrophobic and easily traverse cell membranes. Consequently, classic steroid hormone receptors are located intracellularly and function by regulating gene expression. There is growing evidence of the existence of plasma membrane and juxtamembrane steroid hormone receptors that mediate rapid, nongenomic actions of steroid hormones.

Cell types capable of steroidogenesis are defined as cells that can convert cholesterol to pregnenolone, which represents the initial step in all steroidogenic pathways. Steroidogenic cells have some capacity for cholesterol synthesis but often acquire cholesterol from cholesterol-rich lipoproteins, namely low-density lipoproteins and high-density lipoproteins. Pregnenolone then undergoes further enzymatic modifications. Due to their hydrophobic nature, steroid hormones and precursors can readily exit steroidogenic cells and are not stored. Therefore, the regulation of steroidogenesis occurs at the level of cholesterol uptake, storage, and mobilization, as well as at the level of steroidogenic enzyme gene expression and activity. Steroids are not regulated at the same level as cholesterol.
Gonadotropinsarepituitaryhormones,whichareluteinizinghormoneandfollicle-stimulatinghormone.Thesehormonesareheterodimersthatconsistofacommonα subunitandauniqueβ subunit(see ).Theurineofpostmenopausalwomenisanexcellentsourceofgonadotropinsbecausepostmenopausalserumgonadotropinlevelsarehighasaresultofthelossofnegativefeedbackbyovariansteroids(see ),andthehormonesarefilteredandexcretedasintactmoleculesinurine.Athirdgonadotropinistheplacentalhormonehumanchorionicgonadotropin(hCG;see ),whichhasthesamecommonα subunitandanhCG-specificβ subunit.Itisanextremelystablehormone,andbloodhCGlevelsdoubleevery2daysduringthefirsttrimester.Accordingly,urinarylevelsofhCGalsoincreaserapidly.Pregnancy tests arebasedonimmunologicaldetectioninurineofthehCGspecificβ subunitaspartoftheintacthCGheterodimer.
Steroid hormones are synthesized by a series of enzymatic modifications of cholesterol, and a cyclopentanoperhydrophenanthrene ring (or a derivative thereof) serves as their core (
Fig. 38.8 ). The enzymatic modifications of cholesterol are of three general types: hydroxylation, dehydrogenation/ reduction, and lyase reactions. The purpose of these modifications is to produce a cholesterol derivative that is sufficiently unique to be recognized by a specific receptor. Thus progestins bind to the progesterone receptor, mineralocorticoids bind to the mineralocorticoid receptor, glucocorticoids bind to the glucocorticoid receptor, androgens bind to the androgen receptor, estrogens bind to the estrogen receptor, and the active vitamin D metabolite binds to the vitamin D receptor. The complexity of steroid hormone action is increased by the expression of multiple forms of each receptor. In addition, there is some degree of nonspecificity between steroid hormones and the receptors to which they bind. For example, glucocorticoids bind to the mineralocorticoid receptor with high affinity, and progestins, glucocorticoids, and androgens can all interact with the progesterone, glucocorticoid, and androgen receptors to some degree. As discussed later, steroid hormones are hydrophobic and pass through cell membranes easily. Accordingly, classic steroid hormone receptors are localized intracellularly and act by regulating gene expression. There is mounting evidence of the presence of plasma membrane and juxtamembrane steroid hormone receptors that mediate rapid, nongenomic actions of steroid hormones.
Steroidogenic cell types are defined as cells that can convert cholesterol to pregnenolone, which is the first reaction common to all steroidogenic pathways. Steroidogenic cells have some capacity for cholesterol synthesis but often obtain cholesterol from cholesterol-rich lipoproteins (low-density lipoproteins and high-density lipoproteins). Pregnenolone is then further modified by several enzymatic reactions. Because of their hydrophobic nature, steroid hormones and precursors can leave the steroidogenic cell easily and thus are not stored. Therefore, steroidogenesis is regulated at the level of uptake, storage, and mobilization of cholesterol and at the level of steroidogenic enzyme gene expression and activity. Steroids are not regulated at the •Fig. 38.8 A, Structureofcholesterol,theprecursorofsteroidhormones.B, Structureofsteroidhormones.