Noonan Syndrome: The Hypothalamo-Adrenal and Hypothalamo-Gonadal Axes

Abstract

The hypothalamo-pituitary-adrenal axis has not been studied systematically in Noonan syndrome (NS), despite potential concerns about other aspects of hypothalamo-pituitary function. While adrenarche may be delayed in children with constitutional growth of puberty and in isolated GH deficiency, this does not generally seem to be the case in hypergonadotrophic hypogonadism due to Turner syndrome (TS) and this is (anecdotally) the usual hormonal profile in NS children and adults. Precocious or ‘exaggerated’ adrenarche can be associated with intrauterine growth retardation and is a forerunner of syndrome X. Although NS neonates often have ‘normal’ birth weights, in some it can be artificially inflated by subcutaneous edema (as in TS, where intrauterine growth retardation is characteristic). Overall, however, a controlling role for adrenarche (whether precocious or delayed) in gonadarche in NS seems unlikely. Neither normally descended testes nor normal (even if delayed) pubertal development implies normal fertility in NS men. Interactions between fetal, neonatal, childhood and pubertal testis development and gonadal axis maturation are complex. There is probably a spectrum of abnormalities in NS, but most commonly primary gonadal failure and hypergonadotrophic hypogonadism – characteristic NS molecular genetic abnormalities – may be important for normal germ cell proliferation, development and migration. The identification of different gene defects facilitates understanding of NS phenotypic diversity and provides opportunities for prospective studies on gonadal and adrenal axes in better defined populations less subject to ascertainment bias. At a clinical level, more longitudinal data are still needed with regard to the natural history of pubertal timing, its tempo of progression and the pattern of pubertal growth.