So why is it important?
THG was the anabolic androgenic steroid, also known as The Clear, used by BALCO (Bay Area Laboratory Co-Operative) to provide athletes with a steroid undetectable by doping controls. Once established anabolic steroids became detectable, some athletes and coaches sought to stay ahead of the game.

Patrick Arnold was a 1990 chemistry graduate; he was also a bodybuilder, a powerful combination. He delved deep into the older steroid literature and identified molecules that had already been synthesised and tested, but never marketed, so-called designer steroids.

The three steroids associated with BALCO were Desoxymethyltestosterone (madol), norbolethone (both of which had been previously synthesised but never marketed) and tetrahydrogestinone, aka THG. THG was a totally new molecule. Norbolethone was originally known as "The Clear" as it was believed to be undetectable, since it had never been released onto the commercial market, and no one was on the look out for it. THG had never been made before, so it was even less likely to be detected than norbolethone.

Analysis suggests that THG has very strong anabolic effects:
This conclusion seems justified in view of the musculature of those taking it. It is also credited with considerable androgenic properties. A comparative study of binding of four potent androgens, the natural testosterone together with synthetic dihydrotestosterone, methyltrienolone and THG, showed that THG had the highest affinity for the human androgen receptor. The crystal structures of the human androgenic receptor with three of these, testosterone together with synthetic dihydrotestosterone and THG, revealed that THG makes more van der Waals' contacts with the receptor than do the other steroids, due to the presence of the 17-ethyl and 18-methyl groups, explaining why THG binds approximately twice as strongly to the human androgen receptor than does dihydrotestosterone.

Molecule of the Month Page: THG The illegal performance-enhancing drug used by some athletes and sportspeople:
http://www.chm.bris.ac.uk/motm/THG/THGh.htm

The identification of tetrahydrogestrinone (THG), the first true "designer androgen," as a sports doping agent reflects both an alarmingly sophisticated illicit manufacturing facility and an underground network of androgen abusers in elite sports, as well as the still untapped potential for designer androgens in medicine.

• Never marketed, THG was apparently developed as a potent androgen that was undetectable by conventional International Olympic Committee-mandated urinary sports doping tests.
• As a potent androgen and progestin with unspecified contaminants, its distribution for use at high doses without any prior biological or toxicological evaluation poses significant health risks.
• Yet this diversion of science also highlights the prospect of designer androgens for use in human medicine.
• Designer androgens also offer the possibility of tissue-specific effects enhancing the beneficial effects of androgens while mitigating the undesirable ones.
• Further developments require better understanding of the post receptor tissue selectivity of androgens, comparable to the mechanism underlying that of partial estrogen agonists (SERMs).

This experience highlights the ongoing need for vigilance to detect novel drug doping strategies in order to maintain fairness and safety in elite sports. This will require the deployment of generic catch-all tests, such as sensitive and specific in vitro androgen bioassays, coupled with the development of mass spectrometry-based tests for specific doping agents.

Metabocard for Tetrahydrogestrinone:
https://hmdb.ca/metabolites/HMDB0004626

NPR November 2003:

NPR's Scott Simon talks with Chicago Sun-Times sports columnist Ron Rapoport about the performance-enhancing steroid THG. Until a test for the steroid was developed this past summer, athletes could use the drug without being caught by regular drug testing. It now appears that elite athletes in many sports have used THG.

(2003) Tetrahydrogestrinone, AKA THG:
https://www.npr.org/templates/story/story.php?storyId=1517934

Tetrahydrogestrinone Definition:
An anabolic steroid used to increase muscle growth, developed secretly by attaching four hydrogen atoms to the steroid drug gestrinone for the specific purpose of making its metabolites undetectable in the urine of a person taking it, and used by athletes as a performance enhancer. In 2003 a test was developed for detecting it in urine samples, and several prominent athletes were penalized. THG abbrev.

[From Greek tetra- four + hydor water + gestrinone a drug with a weak progestin and strong anti-progesterone action, used for treating endometriosis]

THCFarmer Forum
https://www.oxfordreference.com/view/10.1093/oi/authority.20110803103256188

(2003) Tetrahydrogestrinone:
https://forums.t-nation.com/t/tetrahydrogestrinone/30695

(2005) Tetrahydrogestrinone (THG) & Erythropoietin (EPO):
https://anabolicminds.com/community/threads/tetrahydrogestrinone-thg-erythropoietin-epo.23570/

• from what I read on it its nothing special, it was just undetected for a while on blood tests so people thought it must be something great, so good for comp guys, the aas that's available already are superior to thg.

(2013) Tetrahydrogestrinone (THG):
https://www.uk-muscle.co.uk/topic/187851-tetrahydrogestrinone-thg/

(2014) The Clear (Tetrahydrogestrinone, THG):
https://www.muscle-research.com/threads/7371-The-Clear-(Tetrahydrogestrinone-THG)

• A synthetic anabolic - androgenic steroid
• Orally active
• Never marketed for medical use
• Has never been fully tested for safety
• Around 10 times more potent than the comparison drugs nandrolone or trenbolone, but with no estrogenic activity
• Was used by a number of high-profile athletes
• Just a couple of drops under the tongue was a sufficient dose

For a time, THG was considered the drug of choice for safe and invisible world record breaking in athletics, being used by several high-profile gold medal winners

The second reported designer anabolic steroid

THG was developed completely in secret as a designer drug, on the basis that doping testers would be unlikely to detect a totally new compound. Patrick Arnold developed a chemical similar to two obscure steroids, norbolethone and desoxymethyltestosterone, which had been reported in scientific literature but never entered mass production, and the banned anabolic steroids trenbolone and gestrinone, the latter of which was used to synthesize it

In 2003, whistleblower Trevor Graham passed a spent syringe containing a small amount of the drug to the United States Anti-Doping Agency. This was then transferred to the research group of pharmacologist Don Catlin, who identified the drug using mass spectrometry techniques and gave it its present name.