Background. Cross-sectional studies from different populations show a variable decline in blood testosterone concentrations as men age. Few population representative cohorts have been followed over time. Objective. To longitudinally quantify the change in serum testosterone and SHBG concentrations with age in two well-defined, representative but geographically widely separated regional Australian cohorts. Subjects and Setting. The Busselton cohort comprises individuals aged 18 to 90 years residing in Western Australia assessed prospectively since 1981. Sera were assayed from 910 men, from whom further samples were available 14 years later in 480. The Dubbo cohort involves individuals aged 61 to 90 years living in Eastern Australia. Baseline sera were collected from 610 men, and additional sera on a second (n = 370) and third (n = 200) occasion from 1989 - 2004. Men from both cohorts are community-dwelling and of predominately European origin.

Results Longitudinal analyses show: (a) total testosterone declines comparably (P > 0.9) by 1.3% (Busselton) and 0.9% (Dubbo) per annum with the same rates of decline when analyses were restricted to men over 60 years of age; (b) annual changes in SHBG were also very similar in age-restricted analyses (2.3% vs 2.5%, P = 0.48); and (c) the annual increase in SHBG was steeper in middle-aged and older men (P < 10^-3 vs young men). These longitudinal changes were all up to 4-fold greater in magnitude compared with cross-sectional analyses of baseline data.

Conclusion In two separate regional Australian populations, blood testosterone fell, and SHBG increased comparably with age. Age-related changes in blood testosterone and SHBG previously described in urban-dwelling men are the same in men who reside in smaller regional cities of another continent.