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. Author manuscript; available in PMC: 2017 Feb 4.
Published in final edited form as: Nat Rev Dis Primers. 2016 Feb 4;2:16003. doi: 10.1038/nrdp.2016.3

Figure 6. Schematic representation of different types of hypogonadism.

Figure 6

a| The normal pituitary–testicular axis. Gonadotropin-releasing hormone (GnRH) stimulates the release of luteinizing hormone (LH). This triggers the testes to respond by producing adequate levels of testosterone, which, in turn, exerts negative feedback control on the hypothalamus and pituitary gland. Both circulating LH and testosterone are in the normal range. b | Secondary hypogonadism. The pituitary release of LH is impaired, the testes are no longer stimulated and testosterone production drops; both circulating LH and testosterone are reduced. c | Subclinical or compensated hypogonadism. The testicular responsiveness to LH is impaired, testosterone production is maintained owing to overstimulation by LH; circulating testosterone is normal or borderline, whereas LH is increased. In this case, the system is driven to its maximal capacity and no further adjustment can be achieved. d | Primary hypogonadism. In testicular failure, increases in LH can no longer sustain testosterone levels: circulating testosterone is low and LH is high.