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Two brothers of Greek Cypriot origin, karyotype 46 XY, aged 17 and 19 years were brought up as females because of predominantly female external genitalia with clitoromegaly and urogenital sinus. At puberty there was genital masculinization with testicular enlargment and descent and development of male musculature and body habitus without gynaecomastia. Both subjects acquired male gender identity and changed from a female to a male gender role. The internal genitalia were normally formed with an ejaculate containing mature spermatozoa. In both subjects plasma testosterone levels were slightly elevated, dihydrotestosterone was in the low normal range and the plasma testosterone to dihydrotestosterone ratio was markedly elevated. Plasma oestrogens were normal whereas SHBG binding capacity was elevated. The urinary 5 beta-aetiocholanolone to 5 alpha-androsterone ratios were elevated compared with normal subjects. Basal plasma LH was normal but, the LH response to LHRH was exaggerated and basal and peak plasma levels of FSH were elevated. 5 alpha-reductase activity in genital skin homogenates was higher when androstenedione was used as a substrate, compared with testosterone, suggesting a decreased affinity of the enzyme for testosterone. The clinical and hormonal features in the two siblings are consistent with an impaired peripheral conversion of testosterone to dihydrotestosterone due to deficiency of the enzyme 5 alpha-reductase.

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