Endocrine Abstracts

Introduction: Although it is well-known that serum testosterone (T) levels decrease in ageing men, to this date no longitudinal data are available on changes in T levels in healthy young adult men.

Objective: To investigate age-related longitudinal changes in serum levels of total T, free T and sex hormone binding globulin (SHBG) in healthy young men.

Methods: Data from 999 healthy men aged 24–46years who participated in a longitudinal population-based sibling-pair study were included. After a mean follow up of 11.6 ± 1.9 (range 7–18) years, 709 participants were re-evaluated. SHBG, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured using Roche e801 immuno-assays. T was measured using LC-MS/MS, free fractions and body mass index (BMI) were calculated. Linear mixed effects modelling was used for longitudinal analysis. Age at baseline and BMI were used as covariates.

Results: Men were 34.5 ± 5.5 years at baseline and 46.4 ± 5.8 years at follow up. During this period BMI increased from 25.1 kg/m² to 26.3 kg/m². At both time points, age and BMI negatively associated with total T and free T (all P < 0.001), and BMI also negatively associated with SHBG (P < 0.001).During follow up, total T levels decreased 14.4% (586.51 ng/dl vs 502.22 ng/dl at baseline and follow-up, respectively, after correction for age at baseline; P < 0.001). The decrease in total T was less pronounced after correction for BMI and changes therein with a decrease of 10.2% (P < 0.001). Free T decreased 17.9% (11.12 ng/dl vs 9.18 ng/dl after correction for age at baseline). The decrease in free T was less pronounced after correction for BMI and changes therein with a decrease of 16.1% (P < 0.001). After adjustment for age and BMI, SHBG increased 5.9% (37.8 nmol/l vs 40.15 nmol/l) , LH increased 6.8% (4.465 U/l vs 4.791 U/l) and FSH increased 15.5% (4.340 U/l vs 5.139 U/l) (allP < 0.001) during follow-up. Further, older age at baseline was associated with larger changes in SHBG (r = 0.115; P = 0.002) and FSH (r = 0.08; P = 0.032).

Conclusion: Already from the 4th decade of life, healthy men experience decreases in total and free T levels, independently from changes in BMI. Given the concurrent rise in gonadotropin levels, the decline in T in our population mostly likely arises from primary testicular dysfunction. Changes in T levels were independent from baseline age, suggesting a similar linear decrease across age within our population.