Incidence of gynaecomastia in Klinefelter syndrome adolescents and outcome of testosterone treatment.

The aim was to define the true incidence of gynaecomastia in adolescent boys with Klinefelter syndrome (KS) and to observe testosterone treatment effects on its duration by examination of the prospectively collected data from a specialist referral clinic for boys with KS, with comparison being made with KS boys identified by a historical newborn chromosome screening programme, together with chromosomally normal controls. Fifty-nine boys over age 13 years were referred to a specialist KS clinic; 21 developed gynaecomastia. The comparator was 14 KS boys identified at birth and 94 chromosomally normal control boys. Testosterone was routinely started at the onset of puberty if gynaecomastia, a manifestation of clinical hypogonadism, was present. Oral or transdermal testosterone was administered in the morning, in a reverse physiological rhythm, and doses were increased according to standard pubertal regimens. The incidence of gynaecomastia was not increased in both the KS cohorts compared with controls. The incidence and age of onset of gynaecomastia was 35.6%, at 12.3 (1.8) years in the KS clinic group; 36.0%, at 13.7 (0.6) years in the newborn survey group; and 34.0%, at 13.6 (0.8) years in the controls. Full resolution of the gynaecomastia occurred in the 12/14 KS clinic boys on testosterone treatment who had completed puberty and as long as adherence was maintained.Conclusion: The incidence of gynaecomastia in KS boys (overall 35.6%) is not increased over typically developing boys. Commencing testosterone when gynaecomastia develops with physiological dose escalation and full adherence can result in the resolution of the gynaecomastia. What is Known: • Gynaecomastia is a common feature in Klinefelter syndrome men. • Hypogonadism occurs from mid-puberty onwards with the absence of the usual rise in testosterone levels. What is New: • The incidence of pubertal gynaecomastia in Klinefelter syndrome is not different from typically developing boys. • Early and prompt starting of testosterone gel treatment and increasing the dose physiologically may help to resolve the gynaecomastia without the need for surgery.