Hypogonadism means diminished functional activity of the gonads —the testicles or the ovaries —that may result in diminished production of sex hormones. Low androgen (e.g., testosterone) levels are referred to as hypoandrogenism and low estrogen (e.g., estradiol) as hypoestrogenism.

Hypergonadotropic hypogonadism (HH), also known as primary or peripheral / gonadal hypogonadism or primary gonadal failure, is a condition which is characterized by hypogonadism which is due to an impaired response of the gonads to the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and in turn a lack of sex steroid production.

Late-onset hypogonadism (LOH) or testosterone deficiency syndrome (TDS) is a condition in older men characterized by measurably low testosterone levels and clinical symptoms mostly of a sexual nature, including decreased desire for mating, fewer spontaneous erections, and erectile dysfunction.