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StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.



Testosterone is the main male hormone responsible for regulating sex differentiation, creating male sex characteristics, spermatogenesis, and also fertility. Testosterone’s effects are an initial seen in the fetus. During the very first 6 main of development, the reproductive organization of males and females are identical. At roughly week 7 in utero, the SRY (sex-related gene ~ above the Y chromosome) initiates the breakthrough of the testicles. Sertoli cell from the testis cords (fetal testicles) eventually develop into seminiferous tubules. Sertoli cells develop a Mullerian-inhibiting substance (MIS), which leads to the regression that the Fallopian tubes, uterus, and upper segment the the vagina (Mullerian frameworks normally existing in females). Fetal Leydig cells and endothelial cell migrate right into the gonad and also produce testosterone, which supports the differentiation of the Wolffian duct (mesonephric duct) frameworks that go on to become the masculine urogenital tract. Testosterone additionally gets convert to dihydrotestosterone (DHT) in the perimeter (discussed below) and also induces the development of the prostate and also male external genitalia. Testosterone is additionally responsible because that testicular descent through the inguinal canal, which wake up in the critical 2 months of fetal development. As soon as an embryo lacks a Y chromosome and thus the SRY gene, ovaries develop. Fetal ovaries carry out not create adequate quantities of testosterone, therefore the Wolffian ducts carry out not develop. There is additionally an lack of MIS in these individuals, causing the breakthrough of the Mullerian ducts and also female reproductive structures. <1>


Testosterone is responsible because that the development of main sexual development, which contains testicular descent, spermatogenesis, enlargement the the penis and testes, and also increasing libido. The testes usually start the descent right into the scrotum roughly 7 months the gestation, as soon as the testes begin secreting reasonable amounts of testosterone. If a male kid is born through undescended but normal testes that carry out not descend through 4 to 6 months of age, management of testosterone can assist the testes descend through the inguinal canals. <2>

Testosterone is likewise involved in regulating second male characteristics, which are those responsible for masculinity. These secondary sex features include male hair patterns, vocal changes, and voice deepening, anabolic effects, which include growth spurts in puberty (testosterone rises tissue growth at the epiphyseal plate at an early stage on and eventual closure of plate later on in puberty) and also skeletal muscle growth (testosterone stimulates protein synthesis). Testosterone also stimulates erythropoiesis, which results in a higher hematocrit in males matches females. Testosterone levels often tend to drop with raising age; due to the fact that of this, men tend to experience a diminish in testicular size, a fall in libido, lower bone density, muscle massive decline, boosted fat production, and decreased erythropoiesis, which leader to possible anemia.


In puberty, the hypothalamic-pituitary-gonadal axis plays a significant role in regulation testosterone levels and gonadal function. The hypothalamus secretes GnRH, i beg your pardon travels under the hypothalamohypophyseal portal device to the anterior pituitary, i beg your pardon secretes luteinizing hormone (LH) and also follicle-stimulating hormone (FSH). LH and FSH room two gonadotropic hormones the travel through the blood and also act on receptors in the gonads. LH, in particular, acts upon the Leydig cell to rise testosterone production. Testosterone borders its very own secretion via an adverse feedback. High level of testosterone in the blood feedback come the hypothalamus to suppress the secretion of GnRH and also feedback to the anterior pituitary, make it less responsive come GnRH stimuli. <3>

Throughout the reproductive life of males, the hypothalamus release GnRH in pulses every 1 come 3 hours. Despite this pulsatile release, however, average plasma level of FSH and LH continue to be fairly continuous from the start of puberty, wherein levels spike, come the third decade that life, where levels peak and slowly begin to decline. Before puberty, testosterone levels room low, showing the low secretion the GnRH and gonadotropins. Changes in neuronal input to the hypothalamus and brain activity throughout puberty cause a dramatic increase in GnRH secretion. 

Leydig cells in the testes duty to revolve cholesterol into testosterone. LH regulates the initial action in this process. Two important intermediates in this procedure are dehydroepiandrosterone (DHEA) and androstenedione. Androstenedione is converted to testosterone by the enzyme 17-beta-hydroxysteroid dehydrogenase. The majority of testosterone is bound come plasma protein such as sex-hormone-binding-globulin and also albumin. This majority supply the protein-bound testosterone acts together a excess of testosterone hormone for the body. The little amounts of cost-free testosterone in the blood act in ~ the level the the tissues, mainly the seminal vesicles, bone, muscle, and also prostate gland. At the to move level, testosterone gets converted to dihydrotestosterone through the enzyme 5-alpha-reductase. Testosterone and dihydrotestosterone have the right to bind to cell receptors and also regulate protein expression. Both men and also women additionally produce weak exhilaration androgens in the zona reticularis that the adrenal glands. These weak-acting androgens are recognized as dehydroepiandrosterone and also androstenedione. They bind to testosterone receptors with weaker affinity but can also be convert to testosterone in the peripheral tissues if produced at high amounts. <4>

Related Testing

Features of testosterone deficiency have the right to be really apparent, i m sorry is why the first steps in diagnosing male hypogonadism indicate adequate history taking and also physical exam. The functions indicative of male hypogonadism can be separated into pre and also post-pubertal. Pre-pubertal features include little testes (less 보다 20 mL in volume), tiny phallus, decreased secondary sex characteristics (e.g., facial or axillary hair), gynecomastia, difficulty gaining muscle mass, eunuchoid proportions, low sperm count, and also low energy/libido. Post-pubertal features encompass those aforementioned (except phallus size and eunuchoid proportions) and osteoporosis and also hot flicker with major hypogonadism. 

If a clinician expects hypogonadism based on history and physical, a full serum testosterone between 8 AM and also 10 AM should be drawn. Typical levels may indicate eugonadal short testosterone. If levels room low, a repeat level must be obtained in addition to FSH and LH levels. Short testosterone in the setting of regular FSH/LH indicates secondary hypogonadism. The next steps would be to get prolactin, T4, 8 am cortisol, iron, and ferritin levels too as brain MRI. Low testosterone in the setup of elevated FSH/LH indicated primary hypogonadism. In the instance of major hypogonadism, a karyotype have to be established.

Hyperandrogenism likewise has assorted clinical presentations, depending upon puberty status and gender. Prepubertal boys v hyperandrogenism may present with virilization. Virilization has penile enlargement, overfill hair growth in androgen-dependent areas, and voice deepening. In prepubertal girls, hyperandrogenism may lead to clitoromegaly, acne, and hirsutism. In adult males, the effects of excess testosterone rely on even if it is the resource is indigenous the adrenals or exogenous. Adrenal androgen elevations have few observable results in males and also do no cause boost in muscle mass or hair growth. In adult females, boosted adrenal androgens deserve to lead come acne, hirsutism, menstrual irregularities, infertility, male-pattern baldness, or virilization. 

Testosterone can be used to treat and manage various medical conditions. Medical conditions in which testosterone deserve to be used encompass metastatic chest cancer, delayed puberty, hypogonadotropic hypogonadism (congenital or acquired), and primary hypogonadism. Toxic impacts of testosterone and synthetic androgens include over-masculinization, hirsutism, reduced menses, acne, and also clitoral enlargement. Rarely, artificial androgens can reason hepatic adenoma, cholestatic jaundice, and also prostatic hypertrophy. Synthetic androgens and also testosterone are contraindicated in pregnancy. 

Androgen antagonists come in different types. GnRH analogs, if provided continuously, have the right to act as medical castration drugs and also are supplied in dealing with prostate cancer. Androgen receptor inhibitors, like flutamide and spironolactone, can be supplied for patients through hirsutism. Steroid synthetic inhibitors, prefer ketoconazole, can be supplied in Cushing disease. 5-alpha reductase inhibitors, choose finasteride, deserve to be used to law benign prostatic hyperplasia. <5>

Clinical Significance

Pathology pertained to testosterone involves either over-production, under-production, receptor insensitivity, or impaired line of testosterone. The following are a couple of of the more common and highly tested testosterone pathologies.

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Over-production that androgens can occur in the following conditions: polycystic ovarian syndrome (PCOS), adrenal virilization/adrenal tumors, ovarian or testicular tumors, Cushing syndrome, and also as a an outcome of exogenous steroid use. To much better understand several of these pathologies the is important to keep in mind the differences between testosterone and dehydroepiandrosterone (DHEA). DHEA is a fairly weak androgen produced by the adrenals and also ovaries/testes. DHEA serves as a precursor for other hormones including testosterone and estrogen. The sulfated kind of DHEA, DHEAS, is details for the adrenal glands. In polycystic ovary syndrome (PCOS), abnormal gonadotropin-releasing hormone (GnRH) secretion leader to an increase in LH secretion. LH stimulates androgen manufacturing by ovarian theca cell which leads to hirsutism, masculine escutcheon, acne and also androgenic alopecia in women affected with PCOS.<6> In adrenal and ovarian tumors, over there is normally rapidly proceeding androgenic symptom (hirsutism, virilization). If testosterone is elevated and also DHEAS is normal, this is most most likely from an ovarian tumor. If DHEAS is elevated and also testosterone is reasonably normal, this is most likely an adrenal tumor. 

Decreased production of testosterone can occur with aging, particular medications, chemotherapy, hypothalamus-pituitary axis disorders, main hypogonadism, cryptorchidism and orchitis, and with hereditary disorders such as Klinefelter and also Kallmann syndrome. Klinefelter syndrome is the most common congenital abnormality that results in primary hypogonadism. In Klinefelter, there is dysgenesis the seminiferous tubules and loss of Sertoli cells which leader to a diminish in inhibin levels and also a resultant increase in FSH. FSH upregulates aromatase leading to raised conversion that androgens come estrogens. In Klinefelter, there is also Leydig cabinet dysfunction which leader to decreased testosterone levels and boost in LH as result of loss of an unfavorable feedback. In Kallmann syndrome, failed migration of GnRH-producing neurons leads to lack of GnRH. No GnRH results in a to decrease in LH, FSH, testosterone, and sperm count. Details to Kallmann syndrome, in comparison to other causes of hypogonadotropic hypogonadism, is defects in the emotion of smell (hyposmia or anosmia). <7>,<8>,<9>

5-alpha reductase is one enzyme that converts testosterone come dihydrotestosterone. Male patients with 5-alpha reductase deficiency present through normal woman or male genitalia or faint genitalia in ~ birth because of lack that dihydrotestosterone. These patients have a male interior urogenital tract (anti-Mullerian hormone is quiet present). At puberty, adolescents with this enzyme deficiency, who may have actually been raised as girls due to lack of an additional male characteristics, begin to develop male an additional sex characteristics and also have main amenorrhea. This patients will have actually normal testosterone and LH, low DHT, and also an increased testosterone-to-DHT ratio. In comparison to 5-alpha reductase deficiency, androgen insensitivity is a condition in which patients lack functional androgen receptors bring about under-virilization. These patients, favor those with 5-alpha reductase deficiency, have actually a 46 XY karyotype. In contrast, however, this patients have actually normal female outside genitalia and usually undescended testes. In adolescence, they experience main amenorrhea and breast advance but have no pubic or axillary hair and also lack the deepening voice transforms that happen with puberty. Lock will have actually a remote vaginal pouch and also abnormal interior reproductive guts (fallopian tubes, uterus, and the upper portion that the vagina) as result of the manufacturing of the Mullerian inhibiting factor. These patients will have actually high level of testosterone and also LH. <10>

Impaired testosterone management can happen in particular cases the congenital adrenal hyperplasia (CAH). In classic CAH (95% that cases), due to 21 hydroxylase deficiency, newborns usually existing with ambiguous genitalia and also later build salt wasting, vomiting, hypotension, and also acidosis. A significant increase in 17-hydroxyprogesterone is diverted in the direction of adrenal androgen synthesis and leads come hyperandrogenism. Hyperandrogenism impairs hypothalamic sensitivity come progesterone bring about a rapid rise in GnRH synthesis and also thus boosted LH and FSH. Elevations in LH and FSH lead to raised gonadal steroid manufacturing (17-hydroxyprogesterone, DHEA, testosterone, LH, and FSH). Diagnosis is with adrenocorticotropic hormone stimulation test reflecting exaggerated 17 hydroxyprogesterone response. <11>