It was stealing his ability to do what he loved - surfing, playing with his kids, walking on the beach. And it was getting worse, quickly. Last year he decided he wanted to end his life before it robbed him entirely of his mobility.
He spent his last moments lying peacefully beside his wife.
“The last moments were beautiful. I didn’t know what to expect, what was going to happen," Ms Thornton says.
"But it was us, laying down on the bed together and I kept thinking, I have to tell him as much as I could possibly think to say. How much I loved him, and the kids loved him and how much I appreciate everything he's done for us."
The journey to that bed in the Swiss clinic, however, was not easy, she said.
As a memorial service is held to farewell her husband on Friday, she hopes that by sharing his journey to access assisted suicide, attitudes in Australia will shift and regulations in Victoria will be made more flexible.
"Every human has the right to choose," Ms Thornton says. "Let's allow people to have some dignity when they die. Troy now died with dignity instead of a cruel end."
The pair had to travel to Switzerland after he was told he wasn't considered as having a terminal illness under Victorian assisting dying laws, which take effect on June 19.
He said he could not find two doctors willing to say with certainty that he would die within 12 months, which was a condition for him to be eligible to access the legislation.
He turned to prominent voluntary euthanasia advocate and founder of Exit International, Philip Nitschke, who helped connect him with Life Circle in Basel.
"It’s not a very nice disease. It’s horrible," Ms Thornton says. "Everything was being taken away bit by bit."
Every human has the right to chooseTroy Thornton's wife Christine
"He called it the beast. He was really inspired by Neale [Daniher], just the way he has done all the fundraising to help his cause and he would say, look at him. I need to suck it up, what have I got to complain about. Then it turned really bad. Everything being taken away bit by bit.”
The couple crowd sourced the funds to get them to Europe and they were unable to take their children.
"That’s why I was so angry," Ms Thornton says.
"To have to travel halfway across the world so Troy could have the right to end his life. I could have gone straight home to my kids. Instead I waited three days before I could see them again."
One of the hardest experiences was watching her children - Laura, 14, and Jack, 17 - say goodbye to their father.
"We have tried to prepare them as much as they can. We have always spoken to kids about stuff, and haven’t kept anything from them, we are upfront and honest," she says.
"It was absolutely horrible [to say goodbye]. It was one of the most gut-wrenching, horrible things. As hard as it was, we know they now remember him as a strong courageous person."
Ms Thornton wants for those who may be in a similar position to her husband to be able to access assisting dying in Australia so they are surrounded by the people they love.
"The current laws are not actually going to cover everyone. A lot of people will miss out. A lot of diseases people have are not going to be covered under that," she says.
"People are suffering, getting to the point where they are losing body functions. In that case, what level of suffering do we need to go through before it can be classed as terminal?"
On Friday, Mr Thornton's colleagues from the CFA, family and friends will gather to farewell him in Mornington.
And even though he may be gone, those invited to the memorial received a personal invitation to the event from Troy himself.
"If you receive this ... my last message ... it’s because I’ve passed and I asked my wife Christine to send this to you from my email account draft box to let you know," the email reads.
"I wanted to thank you all for being my friends and to let you know I am more than happy and in fact feel really blessed and thankful for the full life I have been so fortunate to have had.
"I passed with a full, satisfied heart for my life, my family and friends!
"And ask if you could please get around my wife Christine my son Jack and daughter Laura to support them at this time and not be sad but rather to celebrate the lucky life I feel I’ve had and to help them to move on.
"I just want them to be happy.
"I wish you all the very best and thank you again for your friendship."