title: A population-level decline in serum testosterone levels in American men.
abstract: BACKGROUND
  Age-specific estimates of mean testosterone (T) concentrations appear to vary by year of observation and by birth cohort, and estimates of longitudinal declines in T typically outstrip cross-sectional decreases. These observations motivate a hypothesis of a population-level decrease in T over calendar time, independent of chronological aging.
  
  
  OBJECTIVE
  The goal of this study was to establish the magnitude of population-level changes in serum T concentrations and the degree to which they are explained by secular changes in relative weight and other factors.
  
  
  METHODS
  We describe a prospective cohort study of health and endocrine functioning in randomly selected men of age 45-79 yr. We provide three data collection waves: baseline (T1: 1987-1999) and two follow-ups (T2: 1995-1997, T3: 2002-2004).
  
  
  METHODS
  This was an observational study of randomly selected men residing in greater Boston, Massachusetts.
  
  
  METHODS
  Data obtained from 1374, 906, and 489 men at T1, T2, and T3, respectively, totaling 2769 observations taken on 1532 men.
  
  
  METHODS
  The main outcome measures were serum total T and calculated bioavailable T.
  
  
  RESULTS
  We observe a substantial age-independent decline in T that does not appear to be attributable to observed changes in explanatory factors, including health and lifestyle characteristics such as smoking and obesity. The estimated population-level declines are greater in magnitude than the cross-sectional declines in T typically associated with age.
  
  
  CONCLUSIONS
  These results indicate that recent years have seen a substantial, and as yet unrecognized, age-independent population-level decrease in T in American men, potentially attributable to birth cohort differences or to health or environmental effects not captured in observed data.
authors.affiliation: New England Research Institutes, 9 Galen Street, Watertown, Massachusetts 02472, USA. ttravison@neriscience.com
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Travison
authors.forename: Andre B
authors.initials: AB
authors.lastname: Araujo
authors.forename: Amy B
authors.initials: AB
authors.lastname: O'Donnell
authors.forename: Varant
authors.initials: V
authors.lastname: Kupelian
authors.forename: John B
authors.initials: JB
authors.lastname: McKinlay
mesh_terms: D000367:Age Factors; D000368:Aged; D015331:Cohort Studies; D006801:Humans; D008875:Middle Aged; D011446:Prospective Studies; D013739:Testosterone / Q000097:blood*
chemical_list: D013739:Testosterone