Androstenedione, or 4-androstenedione (abbreviated as A4 or Δ4-dione), also known as androst-4-ene-3,17-dione, is an endogenous weak androgen steroid hormone and intermediate in the biosynthesis of estrone and of testosterone from dehydroepiandrosterone (DHEA).

This rise in androstenedione and DHEA is hypothesized to play a crucial role for learning social, cultural and ecological skills, such as the development and understanding of sexual attraction.

The secondary pathway involves conversion of 17α-hydroxyprogesterone, most often a precursor to cortisol, to androstenedione directly by way of 17,20-lyase.

In premenopausal women, the adrenal glands and ovaries each produce about half of the total androstenedione (about 3 mg/day).

After menopause, androstenedione production is about halved, due primarily to the reduction of the steroid secreted by the ovary.

As is the case for Marisa cornuarietis, freshwater ramshorn snail, where androstenedione is converted into 5α-dihydrotestosterone (DHT) and testosterone (T) in male and into 5α-dihydroandrostenedione (DHA) in females.

Serum levels of androstenedione greater than or equal to 500 ng/dL may indicate the presence of an adrenal or gonadal tumor.

[14] The study also reported that the serum level of estrogens and testosterone produced varied widely between individuals.

[15] The review authors speculate that sufficiently high doses may indeed lead to increased muscle size and strength.

[15] Side effects for women may include the development of male characteristics, clitoromegaly, voice deepening, hirsutism, abnormal menstrual cycles and abnormal bleeding, blood clots, and metabolic disruption based on a study following 10 healthy females administering 100 mg androstenedione.

Sports Illustrated credits Patrick Arnold with introducing androstenedione to the North American market.

Androstenedione was legal and able to be purchased over the counter, and, as a consequence, it was in common use in Major League Baseball throughout the 1990s by record-breaking sluggers like Mark McGwire.

[19] Barry R. McCaffrey, in his capacity as director of the White House Office of National Drug Control Policy from 1996 to 2001, determined that androstenedione could not be classified as an anabolic steroid because there is no proof that it promotes muscle growth.