Steroid isolation , depending on context, is the isolation of chemical matter required for chemical structure elucidation, derivitzation or degradation chemistry, biological testing, and other research needs (generally milligrams to grams, but often more  or the isolation of "analytical quantities" of the substance of interest (where the focus is on identifying and quantifying the substance (for example, in biological tissue or fluid). The amount isolated depends on the analytical method, but is generally less than one microgram.  [ page needed ] The methods of isolation to achieve the two scales of product are distinct, but include extraction , precipitation, adsorption , chromatography , and crystallization . In both cases, the isolated substance is purified to chemical homogeneity; combined separation and analytical methods, such as LC-MS , are chosen to be "orthogonal"—achieving their separations based on distinct modes of interaction between substance and isolating matrix—to detect a single species in the pure sample. Structure determination refers to the methods to determine the chemical structure of an isolated pure steroid, using an evolving array of chemical and physical methods which have included NMR and small-molecule crystallography .  : 10–19 Methods of analysis overlap both of the above areas, emphasizing analytical methods to determining if a steroid is present in a mixture and determining its quantity. 
The vitamin D endocrine system. In this system, the biologically inactive vitamin D 3 is activated, first in the liver to produce 25-hydroxyvitamin D 3 [25(OH)D 3 ], and the endocrine gland (the kidney) converts it to the hormones 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] and 24 R ,25-dihydroxyvitamin D 3 [24 R ,25(OH) 2 D 3 ]. Researchers have identified ≥36 target organs, defined by the presence of the vitamin D receptor (VDR), which is the receptor for the steroid hormone 1α,25(OH) 2 D 3 ; see Table 1 for a list of these target organs. Table 2 lists the 10 extrarenal tissues that investigators have shown possess the 1α-hydroxylase, the enzyme that converts 25(OH)D 3 to the steroid hormone 1α,25(OH) 2 D 3 . Pi, inorganic phosphate.