Dihydrotestosterone
Dihydrotestosterone (DHT, 5α-dihydrotestosterone, 5α-DHT, androstanolone or stanolone) is an endogenous androgen sex steroid and hormone. The enzyme 5α-reductase catalyzes the formation of DHT from testosterone in certain tissues including the prostate gland, seminal vesicles, epididymides, skin, hair follicles, liver, and brain. This enzyme mediates reduction of the C4-5 double bond of testosterone. Relative to testosterone, DHT is considerably more potent as an agonist of the androgen receptor (AR). In addition to its role as a natural hormone, DHT has been used as a medication, for instance in the treatment of low testosterone levels in men ; for information on DHT as a medication, see the androstanolone article. Biological function. DHT is biologically important for sexual differentiation of the male genitalia during embryogenesis, maturation of the penis and scrotum at puberty, growth of facial, body, and pubic hair, and development and maintenance of the prostate gland and seminal vesicles. It is produced from the less potent testosterone by the enzyme 5α-reductase in select tissues, and is the primary androgen in the genitals, prostate gland, seminal vesicles, skin, and hair follicles. DHT signals mainly in an intracrine and paracrine manner in the tissues in which it is  produced, playing only a minor role, if any, as a circulating endocrine hormone. Circulating levels of DHT are 1/10th and 1/20th those of testosterone in terms of total and free concentrations, respectively, whereas local DHT levels may be up to 10 times those of testosterone in tissues with high 5α-reductase expression such as the prostate gland. In addition, unlike testosterone, DHT is inactivated by 3α-hydroxysteroid dehydrogenase (3α-HSD) into the very weak androgen 3α-androstanediol in various tissues such as muscle, adipose, and liver among others, and in relation to this, DHT has been reported to be a  very poor anabolic agent when administered exogenously as a medication. In addition to normal biological functions, DHT also plays an important causative role in a number of androgen-dependent conditions including hair conditions like hirsutism (excessive facial/body hair growth) and pattern hair loss (androgenic alopecia or pattern baldness) and prostate diseases such as benign prostatic hyperplasia (BPH) and prostate cancer. 5α-Reductase inhibitors, which prevent DHT synthesis, are effective in the prevention and treatment of these conditions. Additionally, DHT may play a function in skeletal muscle amino acid transporter recruitment and function. Metabolites of DHT have been found to  act as neurosteroids with their own AR-independent biological activity. 3α-Androstanediol is a potent positive allosteric modulator of the GABA receptor, while 3β-androstanediol is a potent and selective agonist of the estrogen receptor (ER) subtype ERβ. These metabolites may play important roles in the central effects of DHT and by extension testosterone, including their antidepressant, anxiolytic, rewarding/hedonic, anti-stress, and pro-cognitive effects. 5α-Reductase 2 deficiency. Much of the biological role of DHT has been elucidated in studies of individuals with impaired 5α-reductase 2 activity as a result of mutations in the underlying SRD5A2 gene . The condition, known as 5α-reductase 2 deficiency, has a range of presentations as atypical appearances of the external genitalia in males. 5α-Reductase inhibitors. 5α-Reductase inhibitors like finasteride and dutasteride inhibit 5α-reductase type II and/or other isoforms and are able to decrease circulating DHT levels by 65 to 98% depending on the 5α-reductase inhibitor in question. As such, similarly to the case of 5α-reductase type II deficiency, they provide useful insights in the elucidation of the biological functions of DHT. 5α-Reductase inhibitors were developed and are used primarily for the treatment of BPH. The  drugs are able to significantly reduce the size of the prostate gland and to alleviate symptoms of the condition. Long-term treatment with 5α-reductase inhibitors is also able to significantly reduce the overall risk of prostate cancer, although a simultaneous small increase in the risk of certain high-grade tumors has been observed. In addition to prostate diseases, 5α-reductase inhibitors have subsequently been developed and introduced for the treatment of pattern hair loss in men. They are able to prevent further progression of hair loss in most men with the condition and to produce some recovery of hair  in about two-thirds of men. 5α-Reductase inhibitors seem to be less effective for pattern hair loss in women on the other hand, although they do still show some effectiveness. Aside from pattern hair loss, the drugs are also useful in the treatment of hirsutism and can greatly reduce facial and body hair growth in women with the condition. 5α-Reductase inhibitors are overall well tolerated and show a low incidence of adverse effects. Sexual dysfunction, including erectile dysfunction, loss of libido, and reduced ejaculate volume, may occur in 3.4 to 15.8% of men treated with finasteride or dutasteride . A small increase in the risk of affective symptoms including depression, anxiety, and self-harm may be seen. Both the sexual dysfunction and affective symptoms may be due partially or fully to prevention of the synthesis of neurosteroids like allopregnanolone rather necessarily than due to inhibition of DHT production. A very small risk of gynecomastia has been associated with 5α-reductase inhibitors (1.2 to 3.5%). Based on reports of 5α-reductase type II deficiency in males and the effectiveness of 5α-reductase inhibitors for hirsutism in women, reduced body and/or facial hair growth is a likely potential side effect of  these drugs in men. There are very few studies evaluating the side effects of 5α-reductase inhibitors in women. However, due to the known role of DHT in male sexual differentiation, 5α-reductase inhibitors may cause birth defects such as ambiguous genitalia in the male fetuses of pregnant women. As such, they are not used in women during pregnancy. MK-386 is a selective 5α-reductase type I inhibitor which was never marketed. Whereas 5α-reductase type II inhibitors achieve much higher reductions in circulating DHT production, MK-386 decreases circulating DHT levels by 20 to 30%. Conversely, it was found to  decrease sebum DHT levels by 55% in men versus a modest reduction of only 15% for finasteride. However, MK-386 failed to show significant effectiveness in a subsequent clinical study for the treatment of acne. Biological activity. DHT is a potent agonist of the AR, and is in fact the most potent known endogenous ligand of the receptor. It has an affinity (K) of 0.25 to 0.5 nM for the human AR, which is about 2- to 3-fold higher than that of testosterone (K = 0.4 to 1.0 nM) and 15–30 times higher than that of adrenal androgens. In addition , the dissociation rate of DHT from the AR is 5-fold slower than that of testosterone. The EC of DHT for activation of the AR is 0.13 nM, which is about 5-fold stronger than that of testosterone (EC = 0.66 nM). In bioassays, DHT has been found to be 2.5- to 10-fold more potent than testosterone. The elimination half-life of DHT in the body (53 minutes) is longer than that of testosterone (34 minutes), and this may account for some of the difference in their potency. A study of transdermal DHT and testosterone treatment reported terminal half -lives of 2.83 hours and 1.29 hours, respectively. Unlike other androgens such as testosterone, DHT cannot be converted by the enzyme aromatase into an estrogen like estradiol. Therefore, it is frequently used in research settings to distinguish between the effects of testosterone caused by binding to the AR and those caused by testosterone's conversion to estradiol and subsequent binding to and activation of ERs. Although DHT cannot be aromatized, it is still transformed into metabolites with significant ER affinity and activity. These are 3α-androstanediol and 3β-androstanediol, which are predominant agonists of the ERβ. Biochemistry. Biosynthesis . Primary pathway. DHT is synthesized irreversibly from testosterone by the enzyme 5α-reductase. This occurs in various tissues including the genitals (penis, scrotum, clitoris, labia majora), prostate gland, skin, hair follicles, liver, and brain. Around 5 to 7% of testosterone undergoes 5α-reduction into DHT, and approximately 200 to 300 μg of DHT is synthesized in the body per day. Most DHT is produced in peripheral tissues like the skin and liver, whereas most "circulating" DHT originates specifically from the liver. The testes and prostate gland contribute relatively little to concentrations of DHT in circulation. There are two major isoforms  of 5α-reductase, SRD5A1 (type I) and SRD5A2 (type II), with the latter being the most biologically important isoenzyme. There is also third 5α-reductase: SRD5A3. SRD5A2 is most highly expressed in the genitals, prostate gland, epididymides, seminal vesicles, genital skin, facial and chest hair follicles, and liver, while lower expression is observed in certain brain areas, non-genital skin/hair follicles, testes, and kidneys. SRD5A1 is most highly expressed in non-genital skin/hair follicles, the liver, and certain brain areas, while lower levels are present in the prostate, epididymides, seminal vesicles, genital skin, testes, adrenal glands, and kidneys . In the skin, 5α-reductase is expressed in sebaceous glands, sweat glands, epidermal cells, and hair follicles. Both isoenzymes are expressed in scalp hair follicles, although SRD5A2 predominates in these cells. The SRD5A2 subtype is the almost exclusive isoform expressed in the prostate gland. Backdoor pathway. DHT under certain normal and pathological conditions can be produced via a route that does not involve the testosterone intermediate. This route is called the "backdoor pathway". The pathway can be outlined as 17α-Hydroxyprogesterone → 5α-pregnan-17α-ol-3,20-dione → 5α-pregnane-3α,17α-diol-20-one → androsterone → 5α-androstane-3α,17β-diol (androstanediol) → DHT . This pathway is not always considered in the clinical evaluation of patients with hyperandrogenism. Ignoring this pathway may lead to diagnostic pitfalls and confusion, when the conventional androgen biosynthetic pathway cannot fully explain the observed consequences. Distribution. The plasma protein binding of DHT is more than 99%. In men, approximately 0.88% of DHT is unbound and hence free, while in premenopausal women, about 0.47–0.48% is unbound. In men, DHT is bound 49.7% to sex hormone-binding globulin (SHBG), 39.2% to albumin, and 0.22% to corticosteroid-binding globulin (CBG), while in premenopausal women, DHT is bound 78.1–78.4% to SHBG, 21.0–21.3 % to albumin, and 0.12% to CBG. In late pregnancy, only 0.07% of DHT is unbound in women; 97.8% is bound to SHBG while 2.15% is bound to albumin and 0.04% is bound to CBG. DHT has higher affinity for SHBG than does testosterone, estradiol, or any other steroid hormone. Metabolism. DHT is inactivated in the liver and extrahepatic tissues like the skin into 3α-androstanediol and 3β-androstanediol by the enzymes 3α-hydroxysteroid dehydrogenase and 3β-hydroxysteroid dehydrogenase, respectively. These metabolites are in turn converted, respectively, into androsterone and epiandrosterone, then conjugated (via glucuronidation and/or sulfation), released into circulation, and  excreted in urine. Unlike testosterone, DHT cannot be aromatized into an estrogen like estradiol, and for this reason, has no propensity for estrogenic effects. Excretion. DHT is excreted in the urine as metabolites, such as conjugates of 3α-androstanediol and androsterone. Levels. Serum DHT levels are about 10% of those of testosterone, but levels in the prostate gland are 5- to 10-fold higher than those of testosterone due to a more than 90% conversion of testosterone into DHT by locally expressed 5α-reductase. For this reason, and in addition to the fact that DHT is much more potent  as an AR agonist than is testosterone, DHT is considered to be the major androgen of the prostate gland. Medical use. DHT is available in pharmaceutical formulations for medical use as an androgen or anabolic–androgenic steroid (AAS). It is used mainly in the treatment of male hypogonadism. When used as a medication, dihydrotestosterone is referred to as "androstanolone" () or as "stanolone" (), and is sold under brand names such as "Andractim" among others. The availability of pharmaceutical DHT is limited; it is not available in the United States or Canada, but is available in certain European countries. The available formulations  of DHT include buccal or sublingual tablets, topical gels, and, as esters in oil, injectables like androstanolone propionate and androstanolone valerate. Chemistry. DHT, also known as 5α-androstan-17β-ol-3-one, is a naturally occurring androstane steroid with a ketone group at the C3 position and a hydroxyl group at the C17β position. It is the derivative of testosterone in which the double bond between the C4 and C5 positions has been reduced or hydrogenated. History. DHT was first synthesized by Adolf Butenandt and his colleagues in 1935. It was prepared via hydrogenation of testosterone, which had been discovered earlier that  year. DHT was introduced for medical use as an AAS in 1953, and was noted to be more potent than testosterone but with reduced androgenicity. It was not elucidated to be an endogenous substance until 1956, when it was shown to be formed from testosterone in rat liver homogenates. In addition, the biological importance of DHT was not realized until the early 1960s, when it was found to be produced by 5α-reductase from circulating testosterone in target tissues like the prostate gland and seminal vesicles and was found to be more potent than testosterone in bioassays. The biological functions  of DHT in humans became much more clearly defined upon the discovery and characterization of 5α-reductase type II deficiency in 1974. DHT was the last major sex hormone, the others being testosterone, estradiol, and progesterone, to be discovered, and is unique in that it is the only major sex hormone that functions principally as an intracrine and paracrine hormone rather than as an endocrine hormone.