A population-level decline in serum testosterone levels in American men

Context: 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 populationlevel 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. Design: 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). Setting: This was an observational study of randomly selected men residing in greater Boston, Massachusetts. Participants: Data obtained from 1374, 906, and 489 men at T1, T2, and T3, respectively, totaling 2769 observations taken on 1532 men. Main Outcome Measures: 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 populationlevel decrease in T in American men, potentially attributable to birth cohort differences or to health or environmental effects not captured in observed data.