The regulation of gonadotropin secretion in the human male involves a complex interplay between stimulation by GnRH and inhibitory feedback by the sex steroids T and estradiol. In addition, FSH is negatively regulated by a feedback loop mediated by inhibin-B secretion from Sertoli cells (8-11), as well as autocrine/paracrine regulation within the pituitary by activin and follistatin (12). GnRH stimulation of gonadotropin secretion requires pulsatile release from the hypothalamus into the hypophysial portal circulation (13). Although secretion pulses are clearly evident for LH in peripheral blood, FSH pulses are less evident because of their long-circulating half-life (see Chapter 1).
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