CHRISTIAN Salem's overactive thyroid was burning up so much energy, it was equivalent to completing more than 20 hours of exercise per day.

But the Melbourne defender is back playing footy and thriving.

The 27-year-old first noticed something was askew in mid-January, when he started dropping weight in the form of muscle-mass at the peak of pre-season and his heart rate would rise for no reason.

A series of tests followed, before the thought to check his thyroid came, given he had issues with it nine years ago when he was first drafted, and again in 2016.

The thyroid gland helps control body temperature, heart rate, blood pressure and weight.

"I was sort of in and out of hospital a bit, knowing something was going on, but we couldn't really get to the bottom of it. Obviously in that time, that mid-Jan period it's a pretty heavy training phase, so you've got a lot of practice games," Salem told AFL.com.au.

"I was battling to run out some games and I was losing a bit of weight, but it didn't really trigger me at that stage in terms of what was going on. We were doing tests and nothing was coming up, so we added a few extra tests, then I thought of adding the thyroid, just to see how everything's going.

"The level is meant to be between 10 and 18, and it was at 46, which is a pretty high number. I lost about four or five kilos and my heart rate was ridiculous. I had to wear a heart-rate monitor, and if I was sitting on the couch and had to get up to get a glass of water and sit back down, my heart rate would jump to about 125-130 (beats per minute).

"In terms of trying to play a football game, the thyroid is one risk, but having a heart rate that high, from a heart point of view, is not pretty safe.

"It was a pretty challenging period to be honest, because I was told there was no real timeline on it. I've had it before, but the thought of no timeline, just regular blood tests once a week and tracking the levels to make sure it was dropping to a safe level – I was pretty much walking two or three laps with a heart-rate monitor (at training).

"That's how frustrating it was at times, and it wasn't ideal coming off the back of an injury (missing nearly three months with a knee injury) last year. But you've just got to embrace those sorts of challenges that come at you."

Recovery involved lots of rest on the couch watching his teammates play, medication and high-carbohydrate food, as Salem's metabolism kicked into overdrive.

He then suffered a setback with a small meniscus tear in his knee.

"To be honest, I was pretty much on the couch quite a bit, the key was not trying to raise the heart rate too much because as the thyroid specialist – who has been great with me – said, my body was working like it was exercising 20 to 21 hours of the day," Salem said.

"Because it was burning so much, the more I tried to do, or get moving, the more I was stripping weight. So it was plenty of food and plenty of rest for me to just to try and hold some weight.

"I was eating quite a bit, which at the time I was loving, because I was always hungry, my metabolism was so fast. I was also struggling to sleep for a good couple of months, because your body would go from such a high to a low.

"It was a pretty tough time, which probably led to my knee stuff with my muscles not getting back to full strength. But ... it's all part of it, it is what it is. You just got to find a way, and I've had it before. It was just working with the specialists and the docs to try and safely return."

Salem now has three AFL games under his belt this season, and is looking forward to the challenge the ladder-leading Collingwood poses in the King's Birthday clash.

Having been at the club since the beginning of the FightMND Foundation's annual Big Freeze fundraiser (now in its ninth year), Salem said he is privileged to play his small part.

"To see it grow over the time is pretty incredible. It started relatively small as a fundraiser, and to see where it is now in the millions of dollars, it's just unbelievable," Salem said.

"It's just a credit to Neale (Daniher) and the team to be able to raise that. We're pretty honoured to be able to play on such a special occasion. The people who are involved in the Slide every year, and everyone buying in together as a football and probably a [national] community, to be able to share that stage is awesome.

"Neale's been great. Honestly, for someone going through what he's going through, every year he comes in, he just sends such a powerful message, not only to the players, but the staff and the admin and the whole club, who come to listen to him, his positivity and his outlook in life."