Sex Steroids Measurement by Mass Spectrometry in Hypogonadal Men in a Real‐World Setting: Serum Estradiol (E2) and E2 to Total Testosterone Ratio Do Not Differ Between Patients With Klinefelter Syndrome and Non‐Klinefelter Syndrome Hypogonadism

Background Traditionally, Klinefelter syndrome (KS) was considered a clinical condition associated with hyperestrogenism. Aim To compare circulating sex steroids, particularly estrogens, in a cohort of KS and non‐KS hypogonadal men. Materials and Methods A cohort of hypogonadal men, treated and untreated, was prospectively enrolled in a real‐life study at the Endocrinology of Modena. In our clinical lab, serum total testosterone (TT) and estradiol (E2) were assayed twice by two different methods on the same serum sample: immunoassay (IA) with the CMIA D×I800 system (Beckman Coulter) and liquid chromatography–tandem mass spectrometry (LC‐MS/MS) with the Chromsystems MassChrom Steroids. Sex hormone binding globulin (SHBG) was measured by IA for the calculated free testosterone (cFT) by the Vermeulen formula. Results are expressed as median and interquartile ranges in parentheses. Results Note that 203 men (117 KS and 86 non‐KS) were included. Age [KS 32.69 (20.62–41.92); non‐KS 46.62 (34.30–62.52)], body mass index [KS 24.72 (21.93–30.70); non‐KS 28.41 (24.34–31.83) were lower in KS than non‐KS ( p < 0.001). KS were significantly taller (1.80, 1.74–1.84 vs. 1.77, 1.69–1.79, p < 0.001), while weight (82.90, 69.00–100.25 vs. 83.00, 73.00–100.00, p = 0.149) did not differ. Gonadotropins were higher in KS ( p < 0.001). TT, cFT, E2, and SHBG did not differ between KS and non‐KS, both using IA and LC/MS‐MS ( p > 0.05). E2 and E2/TT did not differ between the two groups, irrespective of the assay used. A significant positive correlation was observed between TT concentrations measured by either IA or LC‐MS/MS, in both KS ( R 2 = 0.647, p < 0.001) and non‐KS ( R 2 = 0.820, < 0.001), even when analyzing the correlation between total or calculated free T and E2 measured by LC‐MS/MS in both groups (TT: R 2 = 0.102, p = 0.001 vs. R 2 = 0.219, p < 0.001; cFT: R 2 = 0.162, p < 0.001 vs. R 2 = 0.235, p < 0.001). Conclusions Serum E2 and E2/TT did not differ between KS and non‐KS along the entire spectrum of TT values (from low to normal/high) within the normal range, suggesting no difference in aromatization of estrogen in KS patients.