The term "low testosterone," also known as hypogonadism, refers to a condition in which the body produces an insufficient amount of this hormone. Testosterone is responsible for many male characteristics and functions such as muscle mass, bone density, facial hair growth, and sperm production. When testosterone levels are low, it can lead to various symptoms including decreased libido, erectile dysfunction, fatigue, loss of muscle mass, increased body fat, hot flashes, night sweats, and mood changes such as depression or irritability. Low testosterone is typically caused by an issue with the testicles (primary hypogonadism) or a problem with the hypothalamus or pituitary gland (secondary hypogonadism). Treatment options may include hormone replacement therapy, lifestyle changes, and/or addressing any underlying medical conditions.