Determination of Androgen Bioactivity by an In Vitro Assay During Pubertal Transition in Healthy Boys

OBJECTIVE: To describe the bioactivity of circulating androgens during pubertal transition as determined by an in vitro bioassay assessing androgen receptor (AR) activation, and to compare these findings with circulating concentrations of total testosterone (total T) measured by LC-MS/MS.

METHODS: This longitudinal study included ten healthy boys from the Copenhagen Puberty Study II conducted from 2006 to 2011.

MAIN OUTCOME MEASURE(S): Androgen bioactivity and serum concentrations of total T were measured by an in vitro bioassay and LC-MS/MS with limits of detection of 0.3 and 0.01 nmol/L, respectively. The serum concentration of free testosterone (free T) was calculated using the Vermeulen equation. Pubertal onset was defined as testicular enlargement ≥ 4 mL, assessed by palpation.

RESULTS: Androgen bioactivity was unmeasurable before puberty but measurable in nine of ten boys 0.8-1.7 years after pubertal onset. Total T and free T were measurable prepubertally in all boys and increased in two before pubertal onset. Androgen bioactivity correlated strongly with total T (r = 0.93, p < 0.001) and free T (r = 0.93, p < 0.001). ROC accuracies were 77% for androgen bioactivity, 93% for total T, and 95% for free T.

CONCLUSION: Androgen bioactivity was undetectable before pubertal onset but measurable after in most boys, reflecting both the bioassay's lower sensitivity and potential physiological changes in androgen bioactivity during early puberty. Further studies are needed to clarify these observations.