Vitor Belfort was 33 years old when an Ultimate Fighting Championship doctor in Las Vegas -- whose name has faded from his memory -- diagnosed low testosterone as the cause for his feeling "tired and lethargic." The fix for the two-time champion was a testosterone-replacement therapy regimen that continues to this day.

Now 36, as he basks in a career rebirth that has him set for a spring UFC title fight, Belfort has emerged as the poster child for a practice anti-doping experts portray as, at worst, outright cheating and, at best, an unfair exploitation of a performance-enhancing-drug testing loophole: athletes competing while treated with synthetic testosterone.

Exemptions for testosterone use -- a substance banned in sports as a performance enhancer -- are being handed out at exceedingly high rates in the ever-popular combat sport of mixed martial arts, with state athletic commissions routinely granting allowances based solely on low lab values and diagnoses of hypogonadism, an "Outside the Lines" investigation has found. A major known cause of acquired hypogonadism: prior use of anabolic steroids.

In the past five years, at least 15 mixed martial artists have been issued exemptions to use testosterone, the vast majority revealed or confirmed through public records requests filed by "Outside the Lines" with the major state commissions or athletic bodies overseeing the sport. The sport itself has had more than 20,000 pro fighters over the past five years, according to record keeper mixedmartialarts.com, although fewer than 1,800 MMA combatants are under contract to the sport's dominant promoters -- Zuffa (UFC) and Bellator, which account for 11 of the fighters on TRT. Although only a small fraction, the number of exemptions still dwarfs what can be found in other sports:

• The International Olympic Committee did not issue a single testosterone exemption for the 2012 London Olympics, which featured 5,892 male athletes.

• The U.S. Anti-Doping Agency issued one testosterone exemption last year among the thousands of elite-level athletes under its jurisdiction.

• Major League Baseball has issued six exemptions to athletes over the past six seasons -- an average of 1,200 players populate its rosters each season.

• National Football League officials say testosterone exemptions are "very rare" and only a "handful" have been issued since 1990. Nearly 2,000 players circulate through rosters each season.

• No pro boxer is known to have had an exemption issued through a state athletic commission, and Nevada officials said they have never even received an application.

"It's a huge number," said Dr. Don Catlin, the country's leading anti-doping expert, of the MMA testosterone exemptions. "I am on the IOC committee that reviews [therapeutic-use exemptions for testosterone] requests. We essentially grant none. But in boxing and MMA there is no central control. There is no set of rules that everybody has to follow.

"There is a set of rules for each [state athletic commission], but they are kind of Mickey Mouse rules. So the route to being able to take testosterone is wide open. ... You go in and say 'I have these symptoms.' The doc says, 'Oh yeah, you got low testosterone.' You get a TUE."

Along with exemptions, several MMA fighters and officials also described to "Outside the Lines" widespread use of performance-enhancing substances in the sport. One top contender labeled PED use in the sport "rampant," and a prominent state athletic commission chairman matter-of-factly acknowledged: "We got some doping going on in MMA."

A few state commissions where MMA fights occur less frequently acknowledged they don't test for PEDs or don't require fighters to reveal whether they are being treated with testosterone. Nor, apparently, does any state -- including Nevada, arguably the most influential commission and a model for other regulators -- require notice in a bout agreement of an individual having an exemption to use testosterone, so an opponent is left to learn through the rumor mill, if at all.

Vitor Belfort knocks out Luke Rockhold with a spinning heel kick in their middleweight bout during the UFC on FX event in May 2013 in Brazil. Josh Hedges/Zuffa LLC/Getty Images

Drug testing in MMA is confined to postfight by the state athletic commissions that test for performance-enhancing substances, with Nevada believed to be the only commission attempting out-of-competition testing. The UFC also does some of its own testing, although officials declined comment and little is known about the program. By comparison, major pro leagues such as the NFL and MLB -- in part as a result of urging from Congress -- engage in far more rigorous programs that include testing at the start of camp or spring training as well as year-round, random testing.

"Outside the Lines" found the average age of the MMA fighters when granted their first testosterone exemption was 32 -- the youngest 24. The majority enjoyed exemptions from multiple states, and, in some instances, fighters were found to have simply informed a commission they were on TRT rather than filing a formal application to compete while being treated with testosterone.

U.S. and international anti-doping agencies insist therapeutic-use exemptions for testosterone should be rare and permitted only in dire medical cases such as testicular cancer and Hodgkin's disease, as is the norm in most major sports. The international standard for an exemption specifically states that "low-normal" levels of a hormone isn't justification for granting approval, also noting the same of isolated symptoms such as fatigue, slow recovery from exercise and decreased libido.

Dr. Richard Auchus, a leading endocrinologist and University of Michigan professor of internal medicine, described the incidence of low testosterone or what is known as hypogonadism in healthy 30-year-olds as "vanishingly small" -- or well less than 0.1 percent.

"What people have to understand is a [testosterone exemption] is granted for a disease, not for a [low] lab value," said Auchus, a consultant to USADA. "If you say idiopathic hypogonadotropic hypogonadism, meaning 'I don't know why you have it, but you have low testosterone production and there is nothing wrong with your testes' -- well, that can happen because you are taking exogenous androgen [steroids]. That doesn't cut it."

The issue, said Catlin, is that synthetic testosterone remains one of the favorite drugs to enhance performance. Anti-doping leaders thus fear testosterone exemptions might be used by athletes to dope under the disguise of legitimate medical need.

"It's just a farce that is perpetuated in MMA," said Catlin, who developed the test used to differentiate an individual's natural testosterone from the synthetic version. "It is doping. It is cheating. It is both."

'I feel condemned right now. And I am doing everything legal'

Belfort, dubbed "The Phenom" from the early days of his pro career, says life has never been better, inside or outside the octagon. A sweaty, tightly muscled figure, he chugged from a Muscle Milk bottle and playfully fussed over his two young daughters -- Victoria, 6; and Kyara, 4 -- after a recent grueling gym workout with his Blackzilian fight team in Boca Raton, Fla.

"Eighteen years doing this, my friend," Belfort told a reporter. "Eighteen years -- combat sport. I think you will not find this in history, I believe."

When prodded, Belfort (24-10, winner of eight of his past 10 fights -- seven via KO or TKO) insists the synthetic testosterone regimen hasn't fueled his longevity or late-career revival, which he describes at one point as "devastating people" and "taking guys' heads off." He called the injections a legal, necessary treatment, not an enhancement -- much like insulin for a diabetic. The injections raise his hormone levels back to healthy, normal levels. Without them, he couldn't make a living.

Chael Sonnen, left, who has received a use exemption for testosterone, battles against Jon Jones in their light heavyweight championship bout during the UFC 159 event in April 2013. Al Bello/Zuffa LLC/Getty Images

Belfort, who tested positive for the anabolic steroid 4-Hydroxytestosterone in 2006, cast himself as the most transparent, drug-tested athlete in the sport. "I am telling you, many fighters [are] out there doing drugs, enhancement drugs," he said. "And they don't get tested for it. They don't get tested in camp. I do. ... Other people do TRT and they never go public. I am not ashamed. I am very loyal to my principles. And that is what I do."

"We have, like, tons of fighters with TRT," Belfort said. "It looks like just me. ... I feel condemned right now. And I am doing everything legal."

Yet Belfort, who is training for a late May title fight with middleweight champion Chris Weidman at UFC 173 in Las Vegas, looms ominously over a sport maneuvering through the TRT conundrum. Belfort is expected to appeal to the Nevada State Athletic Commission for an exemption to stay on testosterone therapy for the Weidman fight, which is complicated by the fact that the same commission suspended him in 2006 after a positive steroid test.

Belfort has been a lightning rod, even with his past five fights staged outside the country -- including four in his native Brazil, where he's been allowed to fight under TRT by a Brazilian commission loosely aligned with his UFC promoter. The medical director, Dr. Marcio Tannure, also has been retained independently at times by the UFC. According to Belfort, the Brazilian doctor also has a role in an unrelated DNA study the fighter is participating in.

Dr. Tannure and UFC officials refused multiple interview requests for this story, even after asking for and receiving written questions. When the fighters were receiving testosterone exemptions, 11 of 15 were promoted by Zuffa LLC, which encompasses UFC and Strikeforce. UFC President Dana White has been inconsistent in interviews about the exemptions, saying most recently that they should be banned. But that comment came only a few months after saying the opposite.

White's latest change of heart followed the Association of Ringside Physicians' call last month for the elimination of testosterone exemptions in combat sports -- a motion pushed by Las Vegas-based board member Dr. Margaret Goodman, founder of the Voluntary Anti-Doping Association.