The 300 cutoff for low testosterone came from older men
In US men aged 20 to 44, the bottom of the normal range ran from 409 ng/dL in the early twenties down to 350 by the early forties. Every band sat above 300.
What the study found
The normal floor for men in their twenties is above 400 ng/dL
The 300 ng/dL line deciding who has low testosterone was derived from older men and applied at every age. In a nationally representative US sample of men aged 20 to 44, the bottom of the normal range ran from 409 ng/dL at ages 20 to 24 down to 350 at 40 to 44. All five bands sat above 300.
The caveat · Normal by statistics, not symptoms
Normal here means the middle third of the men surveyed, a statistical convention rather than a line anchored to symptoms or outcomes. Each man gave one measurement, where guidelines require two confirmatory morning readings. And knowing the range shifts with age does not show that treating young men against the higher number would help them; it would mainly widen who counts as deficient.
What we don't know
Normal here means the middle third of the men surveyed, a statistical convention rather than a line anchored to symptoms or outcomes. Each man gave one measurement, where guidelines require two confirmatory morning readings. And knowing the range shifts with age does not show that treating young men against the higher number would help them; it would mainly widen who counts as deficient.