Check out this thread and this post on down if you want more information:
The ability to accurately and routinely measure TT levels was accessible commercially by the late 1970s.
In 1970 just as today, a TT of >=1200 ng/dL for a man not on exogenous testosterone would be rare (top 5 percent) based on the data we have available. But this argument is sometimes used to justify using exogeneous testosterone to run significantly higher than the upper end of a physiologically relevant and reasonable reference range.