Reproductive Endocrinology and Hypogonadism

Reproductive endocrinology concerns the hormonal control of gonadal function through the hypothalamic-pituitary-gonadal axis. Hypogonadism is the deficient production of sex steroids or gametes from the gonads, whether from a problem of the gonads themselves or of their pituitary and hypothalamic control. Because sex steroids are major regulators of bone, this topic connects reproductive endocrinology to mineral and bone metabolism.

Definition

Hypogonadism is a clinical state of reduced sex-steroid production (and often impaired gametogenesis) resulting from disease of the gonads (primary hypogonadism) or from inadequate gonadotrophin stimulation due to hypothalamic or pituitary dysfunction (secondary hypogonadism).

Scope

This entry describes the hypothalamic-pituitary-gonadal axis and the disorder of hypogonadism, distinguishing primary from secondary forms, and emphasises the role of oestrogen and testosterone in maintaining the skeleton. It is included here for its skeletal links and is written as reference material; it does not provide criteria for diagnosing hypogonadism or guidance on hormone therapy in individuals.

Core questions

• How does the hypothalamic-pituitary-gonadal axis regulate sex-steroid production?
• What distinguishes primary from secondary hypogonadism?
• How do oestrogen and testosterone influence the skeleton?
• Why does sex-steroid deficiency contribute to bone loss?

Key concepts

• Hypothalamic-pituitary-gonadal axis
• Gonadotrophin-releasing hormone (GnRH)
• Luteinising and follicle-stimulating hormones
• Primary versus secondary hypogonadism
• Oestrogen and skeletal protection
• Testosterone and bone mass
• Menopausal sex-steroid loss

Mechanisms

The hypothalamus releases gonadotrophin-releasing hormone in pulses, driving pituitary secretion of luteinising and follicle-stimulating hormones, which stimulate the gonads to produce sex steroids and gametes; sex steroids feed back to restrain the axis. In primary hypogonadism the gonads fail and gonadotrophins rise, whereas in secondary hypogonadism the hypothalamus or pituitary under-stimulates the gonads and gonadotrophins are low or inappropriately normal. Sex steroids are important regulators of bone remodelling: oestrogen restrains osteoclastic bone resorption in both sexes, and its loss, as at menopause, accelerates bone loss; testosterone supports bone mass directly and through its conversion to oestrogen. Deficiency of these hormones therefore tilts remodelling toward resorption and predisposes to osteoporosis, linking this topic to mineral and bone metabolism.

Clinical relevance

Reproductive endocrine disorders matter to bone health because sex-steroid deficiency is a recognised contributor to bone loss and fracture risk, and because hormone therapy interacts with skeletal outcomes. This entry frames the axis and the concept of hypogonadism as reference material; it does not advise on testing for or treating hormone deficiency in any individual, which requires clinical evaluation under current guidelines.

Evidence & guidelines

Endocrine Society clinical practice guidelines address testosterone therapy in men with hypogonadism (Bhasin et al., 2018) and the treatment of menopausal symptoms, including the skeletal context of oestrogen loss (Stuenkel et al., 2015); the skeletal consequences are summarised in reviews of osteoporosis (Compston et al., 2019). These define the clinical framework, which the present entry describes rather than prescribes.

History

The elucidation of the hypothalamic-pituitary-gonadal axis, including the pulsatile nature of gonadotrophin-releasing hormone secretion, established the framework for understanding gonadal disorders. Recognition that oestrogen is a dominant regulator of bone in both sexes connected reproductive endocrinology to skeletal health and helped explain the bone loss of menopause and of hypogonadal states more generally.

Key figures

• Shalender Bhasin
• Cynthia Stuenkel
• Frederick Wu
• Sundeep Khosla

Seminal works

• bhasin-2018
• stuenkel-2015

Frequently asked questions

What is the difference between primary and secondary hypogonadism?

In primary hypogonadism the gonads themselves fail, and pituitary gonadotrophins rise in response; in secondary hypogonadism the problem is in the hypothalamus or pituitary, so gonadotrophins are low or inappropriately normal despite low sex steroids.

Why does sex-hormone deficiency affect the skeleton?

Oestrogen and testosterone help restrain bone resorption, so their deficiency, as after menopause or in hypogonadism, accelerates bone loss and increases the risk of osteoporosis, which is why this topic links to mineral and bone metabolism.