Serum estradiol after single dose hCG administration correlates with Leydig cell reserve in hypogonadal men: reassessment of the hCG stimulation test

The hCG stimulation test with assessment of serum testosterone (T) is used for evaluation of testicular function. This retrospective study was undertaken to estimate the diagnostic value of stimulated estradiol (E2) levels in the assessment of Leydig cell function. Serum T and E2 before and after repeated daily hCG injections in 23 adult men with clinically suspected or established primary hypogonadism were studied. After hCG administration serum T increased gradually with peak levels after 72 hours (delta84%, p=0.003). In contrast, serum E2 concentrations reached their maximal levels 24 hours after the first injection (delta168%, p=0.001). Serum T and E2 responses were more attenuated in men with LH < or =17 IU/L as compared to men with lower LH levels. Peak E2 levels after 24 hours correlated significantly with peak T levels after 3 days. We conclude that the increase in serum E2 levels 24 hours after a single hCG injection is an useful additional measure of Leydig cell function. Assessment of E2 increments would render the test procedure more practical, less time-consuming and more cost-effective than assessing peak T levels after 72 hours.