Nice bit of info on Free T, Bio available and SHBG

[Guest guest]

http://www.labtestsonline.org/understanding/analytes/testosterone/faq.html#4

4. What are free and bioavailable testosterone?

Testosterone is present in the blood as " free " testosterone (1-4%) or

bound testosterone. The latter may be loosely bound to albumin, a

serum protein, or bound to a specific binding protein called Sex

Steroid Binding Globulin (SSBG) or Sex Hormone Binding Globulin

(SHBG). The binding between testosterone and albumin is not very

strong and is easily reversed; so the term bioavailable testosterone

(BAT) refers to the sum of free testosterone plus albumin-bound

testosterone. Alternatively, it is the fraction of circulating

testosterone that is not bound to SHBG. It is suggested that BAT

represents the fraction of circulating testosterone that readily

enters cells and better reflects the bioactivity of testosterone than

does the simple measurement of serum total testosterone. Also, varying

levels of SHBG can result in inaccurate measurements of BAT. Decreased

SHBG levels can be seen in obesity, hypothyroidism, androgen use, and

nephritic syndrome. Increased levels are seen in cirrhosis,

hyperthyroidism, and estrogen use. In these situations, measurement of

free testosterone may be more useful. However, technically, free

testosterone is difficult to measure accurately.