A new hand held device is being tested by UK Athletics that help an athlete adjust training according to hormone &/or other bio-markers. Normal tests take a few days to be sent off, analysed & results returned. This can be done on the track! If affective it could be a great way to design your training according to your actual needs & to personalise when & how hard you train. Obviously it's not the final answer as many other factors affect the total training package, but it could be a new tool in the box for trainers.
See the link below for the New Scientist article:

New Scientist article