Quinton Jackson's time at light heavyweight might be over.

"Rampage" told Ariel Helwani on Monday's edition of The MMA Hour that he has diagnosed with an underactive thyroid, which makes him gain weight more rapidly than normal. Jackson said because of that he might never fight at 205 pounds again.

"I've been dealing with this for like a year," Jackson said. "I've been keeping it kind of to myself. It kind of bummed me out. If I don't stay on top of it, I gain weight really fast. That's what's been going on. I've been noticing after my fights I've just been gaining more weight fast, I didn't know what was going on."

An underactive thyroid, or hypothyroidism, can cause obesity, joint pain, infertility and heart disease if it goes untreated, according to the Mayo Clinic. Jackson said he was diagnosed about a year ago, but has probably had the issue for a number of years. "Rampage" said he'll likely be on medication for the condition the rest of his life.

Jackson, 38, is a former UFC light heavyweight champion and has fought almost exclusively at that weight for a decade. Last Friday night, he defeated Satoshi Ishii by unanimous decision in the main event of Bellator MMA: Dynamite 2 at a catchweight of 225 pounds.

"I think I've gotta change my lifestyle, change the way I eat and stuff like that," Jackson said. "Then maybe I can make 205."

Right now, Jackson is looking at potential heavyweight fights against Matt Mitrione or Fedor Emelianenko, whom is a free agent and Bellator is likely interested in bringing in. "Rampage" looks forward to coming back and being in an exciting fight, because he wasn't happy with Ishii's conservative, grinding style.

"I just wish that other fighters shared the same level of intensity and excitement the fans and I did," Jackson said. "I'm not taking anything away from Ishii, even though I don't respect him or the way he fought. But he fought a smart fight. It was really smart. He stayed away from me. He stayed away from me at the right times. He crowded me and got close to me at the right times.

"He knows how to throw punches and stuff like that. He just wanted that win and sometimes guys who just want that 'W,' they don't care how they get it. Me, I want the 'W,' but I want to do it excitingly. I want to like the fans to feel like they got their money work. ... You don't want fans to walk away saying, 'Man, that main event sucked.'"

Jackson (37-11) said his thyroid issue will not affect his fighting, but it might cause some aches and pains during his training. He's not worried about that. "Rampage" is more concerned with the weight aspect of the condition. He'd prefer to continue his career at 205 pounds, because he'd be pretty undersized at heavyweight. But he just isn't sure if hypothyroidism will allow it.

"I don't really want to fight the real big boys, because I noticed that I tried to pick Ishii up at the end of the fight, I was trying to go for a slam and I was like, 'This dude is too big,'" Jackson said. "He felt like he had put some bricks in his ass or something. I don't know if it's his judo background or what. I was like man, maybe I do need to take my ugly ass back down to 205."