An androgen responsible for male secondary sexual characteristics (ie muscle development, body hair growth, voice changes, testicular development).

Testosterone is converted to dihydrotesterone → effects genitals
Testosterone is converted to estradiol → effects bone, sexual function, fat
Testosterone is unconverted → effects muscle mass

Hypogonadism: A decrease in either of two main functions of testes (sperm production or testosterone production)

Increased energy, increased muscle mass, improvement in sexual function (ie libido and erectile function), improved sleep, improved moods

Testosterone at chronically elevated levels (> 1100 ng/dL) increase risk for cardiovascular disease (ie heart attack and stroke), erythrocytosis (too many red blood cells), blood clots, elevated cholesterol (triglycerides), lower HDL, testicular shrinkage, gynecomastia

Testosterone replacement can be administered orally (least effective), topically, intramuscularly (injection) or intranasally (still in research)