Testosterone availability in the blood stream

Testosterone in the blood stream is not all available to the tissues.

There are 3 forms of testosterone in the plasma.

– Some is free in the plasma.

– Some is loosely bound to albumin.

– Some is tihgtly bound to SHBG (sex hormone binding globulin)

As the picture shows, most ie 50+% of testosterone is SHBG bound testosterone and this is not available (it is as if it is stored in a vacuum wrapped pack).

Then around 45% is loosely bound to Albumin with finally 2-3% free.

The free fraction is biologically available. The albumin-bound fraction easily releases free testosterone.

A blood vessel in cross section

Measurements

Total testosterone measures all 3 fractions together.

The free testosterone measurement is usually calculated by using a formula after measuring the SHBG. It is best to measure the free testosterone in older men because age and weight gain increase SHBG production and the total testosterone is less representative of the biological activity than it is in a young man. If we look at the decline in testosterone with age we see that total testosterone falls by around 1% per year whereas free testosterone falls by around 2% per year.

So your doctor should ask for total testosterone, SHBG and then a calculation of free testosterone.