性腺功能低下症，又称性腺机能减退，是指生殖系统的缺憾，导致生殖腺（卵巢或睾丸）的缺乏功能。生殖腺有着两种功能，就是分泌激素（睾酮、雌二醇、抗苗勒氏管激素、黄体素、抑制素B）、激活素及制造生殖细胞（卵子或精子）。缺乏性激素会影响第一性征或第二性征的发育、或会导致成人的戒断效应（如提早的更年期）。卵子或精子的缺憾会导致不育。

性腺功能低下症一般是指永久性的缺憾，而非暂时性或可复原的，且是指生殖激素的缺乏（不论能否生育）。此症甚少表示在正常激素分泌下的不育。

Hypogonadism is a medical term which describes a diminished functional activity of the gonads – the testes and ovaries in males and females, respectively – that may result in diminished sex hormone biosynthesis. In layman's terms, it is sometimes called "interrupted stage 1 puberty". Low androgen (e.g., testosterone) levels are referred to as hypoandrogenism and low estrogen (e.g., estradiol) as hypoestrogenism, and may occur as symptoms of hypogonadism in both sexes, but are generally only diagnosed in males and females respectively. Other hormones produced by the gonads which may be decreased by hypogonadism include progesterone, DHEA, anti-Müllerian hormone, activin, and inhibin. Spermatogenesis and ovulation in males and females, respectively, may be impaired by hypogonadism, which, depending on the degree of severity, may result in partial or complete infertility.