This study showed that
1- Men with higher Body Mass Index (a measure of weight that takes into account height, calculate yours here: http://1.usa.gov/XBIO5K ) had lower testosterone as they aged compared to thinner men. In fact, "a change in BMI from nonobese to obese may be equivalent to a 15 yr fall in T" Note: BMI is a very rough measure that does not take into account muscle or fat mass. A muscular man with little body fat may have high BMI (in my case, my BMI is 30). A better study would measure fat mass, but that tends to be cumbersome and expensive.
2- Smoking did not affect testosterone, but it made the pituitary gland send a higher LH signal to testicles, which could be caused by progressive testicular dysfunction.
3- Not surprisingly, sex hormone binding globulin increased with age. SHBG binds to testosterone and renders it ineffective. Higher insulin levels and lower IGF-1 levels caused by aging may account for this increase in SHBG.
4- Aging causes primary testicular dysfunction with maintained total T and progressively blunted free T associated with higher LH. This interpretation is supported by the age-related attrition of the testicular Leydig cell population and other factors involved with testicular function.
Reference: http://jcem.endojournals.org/content/93/7/2737.long
