The cause of low testosterone is classified as either primary or secondary and further categorized as functional or organic. Primary hypogonadism originates from a problem in the testicles, while secondary hypogonadism indicates a problem in the hypothalamus or the pituitary gland, which are the parts of the brain that signal the testicles to produce testosterone.[1]
Functional hypogonadism is caused by factors that suppress testosterone levels (e.g., medications, obesity, type 2 diabetes, aging, excessive exercise, malnutrition) but are potentially reversible, while organic hypogonadism is caused by a congenital, structural, or destructive condition (e.g., Klinefelter syndrome, Kallmann syndrome, hemochromatosis, inflammatory disease, injury to the testicles, traumatic brain injury, chemotherapy or radiation) that results in largely irreversible hypothalamic, pituitary, or testicular dysfunction.[1]