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Androstenedione (also known as 4-androstenedione and 17-ketoestosterone) is a 19-carbon steroid hormone produced in the adrenal glands and the gonads as an intermediate step in the biochemical pathway that produces the androgen testosterone and the estrogens estrone and estradiol.
Androstenedione is the common precursor of male and female sex hormones. Some androstenedione is also secreted into the plasma, and may be converted in peripheral tissues to testosterone and estrogens.
Androstenedione can be synthesized in one of two ways. The primary pathway involves conversion of 17-hydroxypregnenolone to dehydroepiandrosterone by way of 17,20-lyase, with subsequent conversion of dehydroepiandrosterone to androstenedione via the enzyme 3-β-hydroxysteroid dehydrogenase. The secondary pathway involves conversion of 17-hydroxyprogesterone, most often a precursor to cortisol, to androstenedione directly by way of 17,20-lyase. Thus, 17,20-lyase is required for the synthesis of androstenedione, whether immediately or one step removed.
The production of adrenal androstenedione is governed by ACTH, whereas production of gonadal androstenedione is under control by gonadotropins. 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. Nevertheless, androstenedione is the principal steroid produced by the postmenopausal ovary.
In males, conversion of androstenedione to testosterone requires the enzyme 17β-hydroxysteroid dehydrogenase.
In females, androstenedione is released into the blood by theca cells. Conversion of androstenedione to estrogen (e.g., estrone and estradiol) requires the enzyme aromatase. Androstenedione is a substrate for estrogen production in granulosa cells which produce aromatase. Thus, theca cells and granulosa cells work together to form estrogens.
Androstenedione as a supplement
Androstenedione was manufactured as a dietary supplement, often called andro (or andros) for short. Sports Illustrated credits Patrick Arnold for introducing androstenedione to the North American market. Andro 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. The supplement is banned by the World Anti-Doping Agency, and from the Olympic Games.
The International Olympic Committee in 1997 banned androstenedione and placed it under the category of androgenic-anabolic steroids.
Androstenedione is banned by MLB, the NFL, USOC, NCA, and by the NBA.
Barry R. McCaffrey, the director of the White House's Office of National Drug Control Policy, attempted to determine whether androstenedione could be classified as an anabolic steroid in July 1999. However, he could not because there is no proof of it promoting muscle growth. Now it is known that human bodies require an equal amount of testosterone and androstenedione. The supplement of androstenedione works by increasing the amount of androstenedione which in turn increases the amount of testosterone in the body and then the effects are similar to those of anabolic steroids. 
On March 12, 2004, the Anabolic Steroid Control Act of 2004 was introduced into the United States Senate. It amended the Controlled Substance Act to place both anabolic steroids and prohormones on a list of controlled substances, making possession of the banned substances a federal crime. The law took effect on January 20, 2005. However, androstenedione was legally defined as an anabolic steroid, even though there is scant evidence that androstenedione itself is anabolic in nature.
On April 11, 2004, the United States Food and Drug Administration banned the sale of androstenedione, citing that the drug poses significant health risks commonly associated with steroids. The side effects for men include breast development, behavioral changes, heart disease, and more. Side effects for women are similar to the side effects from anabolic steroids in that their voices will deepen and they may grow facial hair since both occur from an increase level of testosterone. Another side effect of androstenedione is male pattern baldness. The main psychological side effect of androstenedione is depression. Mood swings are also common of any user. 
The "Andro-Project", conducted by medical researchers at East Tennessee State University, showed that the supplement "Andro"(androstenedione/androstenediol) does not increase muscle mass or strength.
Androstenedione has been shown to increase serum testosterone levels over an eight-hour period in men when taken as a single oral dose of 300 mg per day, but a dose of 100 mg had no significant effect on serum testosterone. However, serum levels of estradiol increased following both the 100 mg and 300 mg doses. The study also reported that the serum level of estrogens and testosterone produced varied widely between individuals. A 2006 review paper summarized several studies that examined the effect of androstenedione on strength training. At dosages of 50 mg or 100 mg per day, andro had no effect on muscle strength or size, or on body fat levels. One study used a daily dosage of 300 mg of androstenedione combined with several other supplements, and also found no increase in strength when compared to a control group that did not take the supplements. The review authors speculate that sufficiently high doses may indeed lead to increased muscle size and strength. However, due to the federal ban on androstenedione supplements, it is difficult to carry out new research on its positive and negative effects. The review authors conclude that individuals should not use androstenedione supplements due to the lack of evidence of beneficial effects, the wide variation in individual responses to the supplement, and the risk of unknown side-effects.
Because androstenedione is converted in part to estrogens, people taking this supplement may have estrogenic side-effects, although none of the studies cited above used a sufficiently high dosage to draw any conclusions.
In addition to its role as a precursor to testosterone in the body, androstenedione has androgenic properties in its own right, acting as a weak partial agonist of the androgen receptor. However, in the presence of full agonists like testosterone or dihydrotestosterone (DHT), due to its lower intrinsic activity in comparison, it has antagonistic properties, and can behave more like an antiandrogen like cyproterone.
Conversion of Androstendione to Estrone
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