Now, Testosterone doesn't just float around in your body interacting with everything like Chad at the club. After it's synthesis in the Leydig cells of the balls, testosterone hops a ride on the protein components of the blood. A bit less than 1/3 becomes bound to serum albumin, while the other 2/3 hooks up with creatively named sex hormone-binding globulin (SHBG), with about 2% of testosterone remaining unbound and free in the blood - known as free testosterone, which has the ability to enter into cells and act [2].
Now that we know how testosterone exists in the blood we'll be able to better understand how it's measured. Keep this in mind because different papers use different metrics like:
Bioavaliable Test - same as free test, 2% of total
Total Test - free test + test bound to serum albumin and SHBG
The Landmark Study
Travison, et al. conducted a longitudinal study on 1532 men living in Boston (so we already know their test will be lower than typical ayyy). A longitudinal study is when the same group of men (referred to as a cohort), provides multiple data points over some time course t. In this case, each patient had their serum (total) testosterone and free testosterone measured at 3 periods over the course of about 25 years from 1987-2003.
Now naturally, as you know, testosterone declines as age increases. However, by cleverly utilizing a longitudinal approach to this study the researchers were able to compare different subjects' T levels at the same chronological age. In addition, Travison, et al. trimmed the sample data to reduce individuals with absurdly low or high test (under 100ng/dl or more than 1200ng/dl, respectively), and utilized self-reported metrics of health to further weed out men who developed prostate cancers or other abnormalities that may contribute to changes in T levels.
Metrics used are as follows:
Age
Chronic illnesses including Cancer, Heart disease, Diabetes, etc.
Depression
Prescription meds (by number - an unfortunant weakness of this study)
Education, Marital Status, and Socioeconomic Status
Weight, BMI, and hip-to-waist ratio
Cigarette smoking
Dietary intake (in cals & as grams fat/day)
Sedentary Activity (metric not specified here)