Endocrine Abstracts

Background: The diagnosis of male hypogonadism can be difficult. Free testosterone is calculated if total testosterone is close to the lower limit of normal, however, this is inaccurate in conditions that alter sex-hormone-binding globulin. Alternatively, salivary testosterone is thought to reflect free testosterone, but is present at low concentrations, thus requiring sensitive mass spectrometry instrumentation, and there is limited data on the pre-analytical influences, for example circadian rhythm and effect of meal.

Objective: To investigate circadian rhythm and post-glucose load changes in salivary testosterone.

Methods: Saliva samples were collected from 17 male participants at 09:00, 10:00 and 11:00 following an overnight fast and again at 22:00, 23:00 and midnight following a fast from 14:00. Saliva and serum samples were collected before and after a glucose load from 32 male participants undergoing an oral glucose tolerance test (OGTT). Salivary testosterone assay was setup and validated on the Waters Xevo TQ-S micro-LC-MS/MS.

Results: The salivary testosterone concentration decreased by 12.1% (mean ± SD 23.1 ± 20.9 pmol/l, P=0.0003) at 11:00 and 37.1% (71.0 ± 62.2 pmol/l, P=0.0001) at 22:00 compared to 09:00. There was no significant change between 22:00 and midnight. In the OGTT cohort, the salivary testosterone concentration decreased by 18.1% (32.2 ± 44.6 pmol/l, P=0.0003) at 2 h post-glucose load. The decrease in serum total testosterone or calculated free/bioavailable testosterone at 2 h post-glucose load was not significant. Fasting salivary testosterone correlated more strongly with calculated free testosterone (r= 0.755) and calculated bioavailable testosterone (r= 0.746) compared to serum total testosterone (r= 0.532).

Conclusions: A significant diurnal change and decrease in salivary testosterone after an oral glucose load was identified in two different participant cohorts. Further investigations are required to clarify how much of the decrease was due to circadian change or glucose load.