Through activation of the ARs (as well as the mARs), testosterone has many effects, including the following:
Promotes growth, function, and maintenance of the prostate gland, seminal vesicles, and penis during puberty and thereafter
Promotes growth and maintenance of muscles, particularly of the upper body
Causes subcutaneous fat to be deposited in a masculine pattern and decreases overall body fat
Suppresses breast development induced by estrogens, but can also still produce gynecomastia via excessive conversion into estradiol if levels are too high
Maintains skin health, integrity, appearance, and hydration and slows the rate of aging of the skin, but can also cause oily skin, acne, and seborrhea
Promotes the growth of facial and body hair, but can also cause scalp hair loss and hirsutism
Contributes to bone growth and causes broadening of the shoulders at puberty
Modulates liver protein synthesis, such as the production of sex hormone-binding globulin and many other proteins
Increases production of erythropoietin in the kidneys and thereby stimulates red blood cell production in bone marrow and elevates hematocrit
Exerts negative feedback on the hypothalamic–pituitary–gonadal axis by suppressing the secretion of the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, thereby inhibiting gonadal sex hormone production as well as spermatogenesis and fertility
Regulates the vasomotor system and body temperature via the hypothalamus, thereby preventing hot flashes
Modulates brain function, with effects on mood, emotionality, aggression, and sexuality, as well as cognition and memory
Increases sex drive and erectile capacity and causes spontaneous erections and nocturnal emissions
Increases the risk of benign prostatic hyperplasia and prostate cancer and accelerates the progression of prostate cancer
Decreases breast proliferation and the risk of breast cancer

Pharmacokinetics
Testosterone can be taken by a variety of different routes of administration.
In addition, local levels of DHT in so-called androgenic (5α-reductase-expressing) tissues are also markedly reduced, and this can have a strong impact on certain effects of testosterone. For instance, growth of body and facial hair and penile growth induced by testosterone may be inhibited by 5α-reductase inhibitors, and this could be considered undesirable in the context of, for instance, puberty induction. On the other hand, 5α-reductase inhibitors may prevent or reduce adverse androgenic side effects of testosterone like scalp hair loss, oily skin, acne, and seborrhea. In addition to the prevention of testosterone conversion into DHT, 5α-reductase inhibitors also prevent the formation of neurosteroids like 3α-androstanediol from testosterone, and this may have neuropsychiatric consequences in some men. Aromatase inhibitors
Aromatase inhibitors like anastrozole prevent the conversion of testosterone into estradiol by aromatase. As only a very small fraction of testosterone is converted into estradiol, this does not affect testosterone levels, but it can prevent estrogenic side effects like gynecomastia that can occur when testosterone is administered at relatively high dosages. However, estradiol exerts negative feedback on the hypothalamic–pituitary–gonadal axis and, for this reason, prevention of its formation can reduce this feedback and disinhibit gonadal production of testosterone, which in turn can increase levels of endogenous testosterone. Testosterone therapy is sometimes combined with an aromatase inhibitor for men with secondary hypogonadism who wish to conceive children with their partners. Cytochrome P450 inhibitors
Inhibitors and inducers of cytochrome P450 enzymes like CYP3A4 have been associated with little or no effect on circulating testosterone levels.