Its all about the availability of a T reference standard (calibrant) that CDC released in their effeorts to harmonize T measurements
https://www.cdc.gov/labstandards/hs_standardization.html
The previous ref range that eg LabCorp used was 348 - 1197 ng/dl. Thats based on an LC-MS method using the data from the Framingham study (Bhasin et al, 2011).
The new reference range is 264 - 916 ng/dl. And thats based on the study from Travis et al., 2017 which used data from 1185 healthy nonobese young men. It also used LC-MS technology and used samples from 4 different cohorts the Framingham samples was one cohort).
Now where does the difference come from?
In short, about 90% of the difference comes from the fact that a newly established calibrant was used to calibrate the results from the different studies to the CDC standard.
About 10% comes from the fact that non obese (BMI <30) young males were studied in the Travis study whereas lean young men were used in the Bhasin study.
In more detail. Different testosterone assays have different responses. They give you different results when measuring the same sample. The assays need to be calibrated if one wants to compare different samples from different labs. Thats why the CDC came up with a testosterone assay standardization program. The study by Travis et al is the first study which made use of the newly established CDC calibrant.
Take a look at figure 1 of Travis 2017, second row. The exact same samples from the Framingham study (FHS) were re measured again using the new calibrant and compared to the original results. You can see that an average of difference of 144 ng/dl was found.
The reference range changed predominantly due to the calibration of the T assay to the CDC standars and not because the population testosterone declined.
Google ‘labcorp change testosterone reference range’ and you will find a document entitled ‘q & a testosterone reference interval changes (adult men)’
Why is LabCorp changing the testosterone reference
interval?
A: LabCorp is changing to the recently standardized reference
interval for adult males based upon testosterone assays
standardized to the CDC reference method. This change
was driven by the consensus effort for accurate testosterone
testing, which was endorsed by a group of professional
associations, government agencies, and commercial entities
in 2010.1