The Study

Researchers at the University of Miami tracked testosterone levels in 4,045 American men aged 15–39 using the CDC's own national health database (NHANES). They looked at data spanning 1999 to 2016 — nearly two decades.

What they found

• Average testosterone dropped from 605 ng/dL in 1999 to 451 ng/dL by 2016 — a 25% decline in 15 years

• 1 in 5 young men (20%) met the clinical threshold for testosterone deficiency

• The decline held even in men with normal body weight — meaning this isn't just an obesity story

• Lower physical activity correlated with lower testosterone, but didn't fully explain the trend either

• The researchers couldn't pin it on any single cause — diet, environmental toxins, and endocrine disruptors are all flagged as possible contributors

Source

• Published in European Urology Focus (European Association of Urology journal)

• CDC data — not a pharma-funded study

• 75 citations, 88 captures on Scopus

• Lead institution: University of Miami Miller School of Medicine

Access the journal here: https://pubmed.ncbi.nlm.nih.gov/32081788/

Alternatively, read the full article via Sci-Hub: https://sci-hub.st/10.1016/j.euf.2020.02.006