Hypogonadotropic hypogonadism (HH),  is due to problems with either the hypothalamus or pituitary gland affecting the hypothalamic-pituitary-gonadal axis (HPG axis). Hypothalamic disorders result from a deficiency in the release of gonadotropic releasing hormone (GnRH), while pituitary gland disorders are due to a deficiency in the release of gonadotropins from the anterior pituitary. GnRH is the central regulator in reproductive function and sexual development via the HPG axis. GnRH is released by GnRH neurons, which are hypothalamic neuroendocrine cells, into the hypophyseal portal system acting on gonadotrophs in the anterior pituitary. The release of gonadotropins, LH and FSH, act on the gonads for the development and maintenance of proper adult reproductive physiology. LH acts on Leydig cells in the male testes and theca cells in the female. FSH acts on Sertoli cells in the male and follicular cells in the female. Combined this causes the secretion of gonadal sex steroids and the initiation of folliculogenesis and spermatogenesis. The production of sex steroids forms a negative feedback loop acting on both the anterior pituitary and hypothalamus causing a pulsatile secretion of GnRH. GnRH neurons lack sex steroid receptors and mediators such as kisspeptin stimulate GnRH neurons for pulsatile secretion of GnRH. Types
There are two subtypes of HH, congenital HH (CHH) and acquired HH (AHH). CHH is due to genetic abnormalities resulting in non-functional GnRH secreting neurons or gonadotropic cell dysfunction in the anterior pituitary. CHH is divided into 2 subtypes depending on the condition of the olfactory system, anosmic HH (Kallman syndrome) and normosmic HH.
Gonadotropin therapy in HH men usually  is able to generate enough sperm for fertility to occur, however sperm count is still lower than normal.In the female, the goal for gonadotropin therapy is to obtain ovulation. This is obtained with FSH treatment followed by hCG or LH to trigger ovulation. FSH will stimulate granulosa cells for follicular maturation while LH will act on luteal cells to produce steroids aiding follicular maturation and preparing the endometrium for pregnancy.For hyperprolactinaemia-caused AHH, dopamine agonists are used to improve GnRH secretion. Dopamine binds to D2 receptors on lactotrophs within the anterior pituitary. This results in the inhibition of secretion of prolactin resulting in less direct and indirect inhibition of GnRH secretion.In up to 10–20% of cases, patients can exhibit sustained fertility and steroid production after therapy, resulting in hypogonadotropic hypogonadism reversal. The mechanism for this reversal is unknown but there is believed to be some neuronal plasticity within GnRH releasing cells. See also
Androgens and estrogens
GnRH and gonadotropins (FSH and LH)
Hypergonadotropic hypogonadism
Hypothalamic–pituitary–gonadal axis
Isolated hypogonadotropic hypogonadism

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