"Testosterone within the circulation is principally bound to proteins, the most important of which is sex hormone binding globulin (SHBG). Only about 2 percent of testosterone is unbound (bio-available) and therefore free to enter cells in order to effect its biological actions by binding to androgen receptors...."
"Hypotestosteronaemia levels [that is, low levels] quoted in papers
vary between around 300 ng/dl (10.4 nmol/L) and 400 ng/dl (13.9
"Bioavailable testosterone refers to the 'free' portion of the hormone unbound to carrier proteins which is able to act directly upon target tissues."
"It is well known that androgen levels decline in men with age.
Furthermore, as a result of the increase in binding capacity for
testosterone in serum, there is an early and more dramatic decline in
free testosterone, the physiologically functioning component of serum
testosterone, than in the total testosterone concentration...."
"The ideal therapy should consist of creating physiologic testosterone concentrations (400 to 700 ng/dL in blood taken in the morning) and restoring circadian variations."
"Of great interest is a recent finding that men with coronary artery
disease have significantly lower levels of plasma testosterone than
controls and that intracoronarv testosterone infusion in men with
coronary artery disease increases coronary blood flow. Such findings
suggest that age related declines in testosterone in men may be
associated with heart disease ...."
Free Androgen Index (FAI): reference range is 1.6 to 2.9
"The FAI is the ratio of total testosterone to SHBG. It provides a
simple assessment of physiologically active testosterone.
FAI = 100 x TT (in nmol/L) / SHBG (in nmol/L)"
Normal SHBG levels vary widely: range at 13 to 71 nmol/L
Free Testosterone (Bioavailable Testosterone):
Adult Male Reference Interval
Free Testosterone: 47.0-244.0 pg/mL
% Free Testosterone: 1.6 - 2.9%
20 - 39 years: 400 - 1080 ng/dL
40 - 59 years:
350 - 890 ng/dL
60 years and over:
350 - 720 ng/dL
Sex Hormone Binding Globulin [SHBG]:
13 - 15 years
13 - 63 nmol/L
16 - 18 years
11 - 54 nmol/L
19 - 150 years
13 - 71 nmol/L
LABORATORY SERVICES HANDBOOK