Testosterone is the primary male hormone responsible for regulating sex differentiation, producing male sex characteristics, spermatogenesis, and fertility. Testosterone’s effects are first seen in the fetus. During the first 6 weeks of development, the reproductive tissues of males and females are identical. At around week 7 in utero, the SRY (sex-related gene on the Y chromosome) initiates the development of the testicles. Sertoli cells from the testis cords (fetal testicles) eventually develop into seminiferous tubules. Sertoli cells produce a Mullerian-inhibiting substance (MIS), which leads to the regression of the Fallopian tubes, uterus, and upper segment of the vagina (Mullerian structures normally present in females). Fetal Leydig cells and endothelial cells migrate into the gonad and produce testosterone, which supports the differentiation of the Wolffian duct (mesonephric duct) structures that go on to become the male urogenital tract. Testosterone also gets converted to dihydrotestosterone (DHT) in the periphery (discussed below) and induces the formation of the prostate and male external genitalia. Testosterone is also responsible for testicular descent through the inguinal canal, which occurs in the last 2 months of fetal development. When an embryo lacks a Y chromosome and thus the SRY gene, ovaries develop. Fetal ovaries do not produce adequate amounts of testosterone, thus the Wolffian ducts do not develop. There is also an absence of MIS in these individuals, leading to the development of the Mullerian ducts and female reproductive structures. 
Testosterone is responsible for the development of primary sexual development, which includes testicular descent, spermatogenesis, enlargement of the penis and testes, and increasing libido. The testes usually begin the descent into the scrotum around 7 months of gestation, when the testes begin secreting reasonable quantities of testosterone. If a male child is born with undescended but normal testes that do not descend by 4 to 6 months of age, administration of testosterone can help the testes descend through the inguinal canals. 
Testosterone is also involved in regulating secondary male characteristics, which are those responsible for masculinity. These secondary sex characteristics include male hair patterns, vocal changes, and voice deepening, anabolic effects, which include growth spurts in puberty (testosterone increases tissue growth at the epiphyseal plate early on and eventual closure of plate later in puberty) and skeletal muscle growth (testosterone stimulates protein synthesis). Testosterone also stimulates erythropoiesis, which results in a higher hematocrit in males versus females. Testosterone levels tend to drop with increasing age; because of this, men tend to experience a decrease in testicular size, a drop in libido, lower bone density, muscle mass decline, increased fat production, and decreased erythropoiesis, which leads to possible anemia.
In puberty, the hypothalamic-pituitary-gonadal axis plays a major role in regulating testosterone levels and gonadal function. The hypothalamus secretes GnRH, which travels down the hypothalamohypophyseal portal system to the anterior pituitary, which secretes luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are two gonadotropic hormones that travel through the blood and act on receptors in the gonads. LH, in particular, acts on the Leydig cells to increase testosterone production. Testosterone limits its own secretion via negative feedback. High levels of testosterone in the blood feedback to the hypothalamus to suppress the secretion of GnRH and also feedback to the anterior pituitary, making it less responsive to GnRH stimuli. 
Throughout the reproductive life of males, the hypothalamus releases GnRH in pulses every 1 to 3 hours. Despite this pulsatile release, however, average plasma levels of FSH and LH remain fairly constant from the start of puberty, where levels spike, to the third decade of life, where levels peak and slowly begin to decline. Prior to puberty, testosterone levels are low, reflecting the low secretion of GnRH and gonadotropins. Changes in neuronal input to the hypothalamus and brain activity during puberty cause a dramatic rise in GnRH secretion.
Leydig cells in the testes function to turn cholesterol into testosterone. LH regulates the initial step in this process. Two important intermediates in this process are dehydroepiandrosterone (DHEA) and androstenedione. Androstenedione is converted to testosterone by the enzyme 17-beta-hydroxysteroid dehydrogenase. The majority of testosterone is bound to plasma proteins such as sex-hormone-binding-globulin and albumin. This majority supply of protein-bound testosterone acts as a surplus of testosterone hormone for the body. The small amounts of free testosterone in the blood act at the level of the tissues, primarily the seminal vesicles, bone, muscle, and prostate gland. At the cellular level, testosterone gets converted to dihydrotestosterone by the enzyme 5-alpha-reductase. Testosterone and dihydrotestosterone can bind to cell receptors and regulate protein expression. Both men and women also produce weak acting androgens in the zona reticularis of the adrenal glands. These weak-acting androgens are known as dehydroepiandrosterone and androstenedione. They bind to testosterone receptors with weaker affinity but can also be converted to testosterone in the peripheral tissues if produced at high amounts. 
Features of testosterone deficiency can be very apparent, which is why the first steps in diagnosing male hypogonadism involve adequate history taking and physical exam. The features indicative of male hypogonadism can be divided into pre and post-pubertal. Pre-pubertal features include small testes (less than 20 mL in volume), small phallus, decreased secondary sex characteristics (e.g., facial or axillary hair), gynecomastia, difficulty gaining muscle mass, eunuchoid proportions, low sperm count, and low energy/libido. Post-pubertal features include those previously mentioned (except phallus size and eunuchoid proportions) as well as osteoporosis and hot flashes with severe hypogonadism.
If a clinician expects hypogonadism based on history and physical, a total serum testosterone between 8 AM and 10 AM should be drawn. Normal levels may indicate eugonadal low testosterone. If levels are low, a repeat level should be obtained along with FSH and LH levels. Low testosterone in the setting of normal FSH/LH indicates secondary hypogonadism. The next steps would be to get prolactin, T4, 8 AM cortisol, iron, and ferritin levels as well as brain MRI. Low testosterone in the setting of elevated FSH/LH indicated primary hypogonadism. In the case of primary hypogonadism, a karyotype should be established.
Hyperandrogenism also has various clinical presentations, depending on puberty status and gender. Prepubertal boys with hyperandrogenism may present with virilization. Virilization includes penile enlargement, excess 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 depend on whether the source is from the adrenals or exogenous. Adrenal androgen elevations have few observable effects in males and do not cause an increase in muscle mass or hair growth. In adult females, increased adrenal androgens can lead to 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 can be used include metastatic breast cancer, delayed puberty, hypogonadotropic hypogonadism (congenital or acquired), and primary hypogonadism. Toxic effects of testosterone and synthetic androgens include over-masculinization, hirsutism, decreased menses, acne, and clitoral enlargement. Rarely, synthetic androgens can cause hepatic adenoma, cholestatic jaundice, and prostatic hypertrophy. Synthetic androgens and testosterone are contraindicated in pregnancy.
Androgen antagonists come in different types. GnRH analogs, if given continuously, can act as medical castration drugs and are used in treating prostate cancer. Androgen receptor inhibitors, like flutamide and spironolactone, can be used for patients with hirsutism. Steroid synthesis inhibitors, like ketoconazole, can be used in Cushing disease. 5-alpha reductase inhibitors, like finasteride, can be used to treat benign prostatic hyperplasia. 
Pathology related to testosterone involves either over-production, under-production, receptor insensitivity, or impaired metabolism of testosterone. The following are a few of the more common and highly tested testosterone pathologies.
Over-production of androgens can occur in the following conditions: polycystic ovarian syndrome (PCOS), adrenal virilization/adrenal tumors, ovarian or testicular tumors, Cushing syndrome, and as a result of exogenous steroid use. To better understand some of these pathologies it is important to note the differences between testosterone and dehydroepiandrosterone (DHEA). DHEA is a relatively weak androgen produced by the adrenals and ovaries/testes. DHEA serves as a precursor for other hormones including testosterone and estrogen. The sulfated form of DHEA, DHEAS, is specific for the adrenal glands. In polycystic ovary syndrome (PCOS), abnormal gonadotropin-releasing hormone (GnRH) secretion leads to an increase in LH secretion. LH stimulates androgen production by ovarian theca cells which leads to hirsutism, male escutcheon, acne and androgenic alopecia in women affected with PCOS. In adrenal and ovarian tumors, there is usually rapidly progressing androgenic symptoms (hirsutism, virilization). If testosterone is elevated and DHEAS is normal, this is most likely from an ovarian tumor. If DHEAS is elevated and testosterone is relatively normal, this is most likely an adrenal tumor.
Decreased production of testosterone can occur with aging, certain medications, chemotherapy, hypothalamus-pituitary axis disorders, primary hypogonadism, cryptorchidism and orchitis, and with genetic disorders such as Klinefelter and Kallmann syndrome. Klinefelter syndrome is the most common congenital abnormality that results in primary hypogonadism. In Klinefelter, there is dysgenesis of seminiferous tubules and loss of Sertoli cells which leads to a decrease in inhibin levels and a resultant increase in FSH. FSH upregulates aromatase leading to increased conversion of androgens to estrogens. In Klinefelter, there is also Leydig cell dysfunction which leads to decreased testosterone levels and an increase in LH due to loss of negative feedback. In Kallmann syndrome, failed migration of GnRH-producing neurons leads to lack of GnRH. No GnRH results in a decrease in LH, FSH, testosterone, and sperm count. Specific to Kallmann syndrome, in comparison to other causes of hypogonadotropic hypogonadism, is defects in the sensation of smell (hyposmia or anosmia). ,,
5-alpha reductase is an enzyme that converts testosterone to dihydrotestosterone. Male patients with 5-alpha reductase deficiency present with normal female or male genitalia or ambiguous genitalia at birth due to lack of dihydrotestosterone. These patients have a male internal urogenital tract (anti-Mullerian hormone is still present). At puberty, adolescents with this enzyme deficiency, who may have been raised as girls due to lack of secondary male characteristics, begin to develop male secondary sex characteristics and have primary amenorrhea. These patients will have normal testosterone and LH, low DHT, and an increased testosterone-to-DHT ratio. In contrast to 5-alpha reductase deficiency, androgen insensitivity is a condition in which patients lack functional androgen receptors resulting in under-virilization. These patients, like those with 5-alpha reductase deficiency, have a 46 XY karyotype. In contrast, however, these patients have normal female external genitalia and usually undescended testes. In adolescence, they experience primary amenorrhea and breast development but have no pubic or axillary hair and lack the deepening voice changes that occur with puberty. They will have a blind vaginal pouch and abnormal internal reproductive organs (fallopian tubes, uterus, and the upper portion of the vagina) due to the production of the Mullerian inhibiting factor. These patients will have high levels of testosterone and LH. 
Impaired testosterone metabolism can occur in certain cases of congenital adrenal hyperplasia (CAH). In classic CAH (95% of cases), due to 21 hydroxylase deficiency, newborns usually present with ambiguous genitalia and later develop salt wasting, vomiting, hypotension, and acidosis. A marked increase in 17-hydroxyprogesterone is diverted towards adrenal androgen synthesis and leads to hyperandrogenism. Hyperandrogenism impairs hypothalamic sensitivity to progesterone leading to a rapid rise in GnRH synthesis and thus increased LH and FSH. Elevations in LH and FSH lead to increased gonadal steroid production (17-hydroxyprogesterone, DHEA, testosterone, LH, and FSH). Diagnosis is with adrenocorticotropic hormone stimulation test showing exaggerated 17 hydroxyprogesterone response.