-How does testosterone live in your blood? -
Gentlemen (AND ladies here too!), if you are going to request testosterone testing from your doctor then you should understand how testosterone exists in your body.
Because testosterone is derived from a fat (remember, made from cholesterol!), then it doesn’t travel alone very well in your blood (water and oil don’t mix well). So your liver and testes make protein (a water-soluble protein) to help transport testosterone throughout the body.
Albumin Bound: This testosterone is bioavailable, but not free. Meaning it can’t directly enter the cell because the albumin protein is holding it back. Reason why? Albumin (unlike SHBG) does not bind very strongly to testosterone, but enough to keep it floating in the blood. So technically it is bio-available but not active until it’s needed.
SHBG-Bound: Like albumin, SHBG is a protein but it binds very strongly to testosterone particles (and DHT). This helps it translocate around the body. However, because SHBG has a high affinity for testosterone, this testosterone is NOT bioavailable, and can’t enter the cell unless something removes the SHBG attached.
Free testosterone (circled): this is the testosterone that is active, free and able to enter the cell and bind to testosterone (androgen receptors). This is one of the most important markers because this is where all the action and activity happens! So if your free T is down, then symptoms - low libido, erectile dysfunction, fatigue, low muscle mass - show up!
Total testosterone: In the pie chart, if you take the total of SHBG-bound, Albumin-bound, and free testosterone, then you have your total testosterone.
Bhasin S, Cunningham GR, Hayes FJ, et al. Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2010;95(6):2536-2559. doi:10.1210/jc.2009-2354
Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. doi:10.1210/jc.2018-00229