wrote:

The difference is small (~3 parts per thousand PDB), but readily
measurable if you've got good technique. It arise from the fact that
synthetic testosterone is produced from plant sterols, which are lower
in 13C than animal hormones/tissues/etc. due to isotopic
discrimination.

[continued searching for possible Floyd excuse below]
1. What if you had been vegetarian for some relatively long period of
time. Would this mean that the carbon entering your body had less 13C?
Would this lead to your testosterone having less 13C?

2. I doubt Floyd's a vegetarian, but perhaps he is -- he does live in
California, after all. [tired of people blaming his quirks on having
been raised a Mennonite and looking for different scapegoat]