As men age, circulating free testosterone decreases as they produce less testosterone while producing more estradiol (an estrogen found in men). Every man will experience a decrease in "bioavailable" testosterone, but for reasons still unknown, some men’s hormone levels drop lower than others.
A man whose total blood serum testosterone levels fall below 300 nanograms per deciliter has a high likelihood of experiencing hypogonadal symptoms. Free testosterone levels below 5.0 picograms per milliliter can also trigger symptoms. Psychological stress, alcohol, injuries or surgery, medications, obesity and infections can also contribute to its onset.