Hypogonadism is a medical term for a defect of the reproductive system which results in lack of function of the gonads (ovaries or testes). The gonads have two functions: to produce hormones (testosterone, estradiol, antimullerian hormone, progesterone, inhibin B), activin and to produce gametes (eggs or sperm). Deficiency of sex hormones can result in defective primary or secondary sexual development, or withdrawal effects (e.g., premature menopause) in adults. Defective egg or sperm development results in infertility.

The term hypogonadism is usually applied to permanent rather than transient or reversible defects, and usually implies deficiency of reproductive hormones, with or without fertility defects. The term is less commonly used for infertility without hormone deficiency.

There are many possible types of hypogonadism and several ways to categorize them.

For example, hypogonadism can be congenital (a condition present at birth) or acquired (a condition which develops in childhood or adult life). An example of congenital hypogonadism is Turner syndrome and an example of acquired hypogonadism is castration.

Hypogonadism can involve just hormone production or just fertility, but most commonly involves both. Examples of hypogonadism that affect hormone production more than fertility are hypopituitarism and Kallmann syndrome; in both cases fertility is reduced until hormones are replaced but can be achieved solely with hormone replacement. Examples of hypogonadism that affect fertility more than hormone production are Klinefelter syndrome and Kartagener syndrome.

Hypogonadism is also categorized by endocrinologists by the level of the reproductive system which is defective.

• Hypothalamic (e.g., Kallmann syndrome)
• Pituitary (e.g., hypopituitarism)
• Gonadal (e.g., Klinefelter syndrome, Turner syndrome)
• Hormone response (e.g., androgen insensitivity syndrome)

Hypogonadism resulting from hypothalamic or pituitary defects is termed secondary hypogonadism or central hypogonadism (referring to the central nervous system). Hypogonadism resulting from defects of the gonads is traditionally referred to as primary hypogonadism.

Physicians measure gonadotropins (LH and FSH) to distinguish primary from secondary hypogonadism. In primary hypogonadism the LH and/or FSH are usually elevated, while in secondary hypogonadism both are normal or low.

Hypogonadism is often discovered during evaluation of delayed puberty, but ordinary delay which eventually results in normal pubertal development and reproductive function is termed constitutional delay .

Hypogonadism is most often treated by replacement of the appropriate hormones. For men this is testosterone. For women estradiol and progesterone are replaced. Some types of fertility defects can be treated; some cannot.