For Thorn, a naturally strong boy when he was growing up in Otago, something clicked when Lindsay, a watchmaker, passed away and he began training like never before. Now, at 40, he has faced up to retirement and he will hang the boots up at the end of Leicester's season.
Bradley Cornegie Thorn was born in Mosgiel in February 1974. After a stint in the town south of Dunedin, and a shorter stint in Queenstown, the family - dad, mother Robin and brother Aaron - moved to the brighter lights of Brisbane.
Thorn, then 9, had played schoolboy rugby but his heroes would soon be Australian league players. A mere 10 years later he would be a professional league player himself.
"I had a state forest near my house [in Brisbane], that's where I first learned about training," he told an interviewer while at the Crusaders four years ago. "I had a mindset where I wanted to feel like peeing, pooing and spewing all at once at the end.
"At five o'clock I would go and do this run where I would jog anything flat but anything uphill I would sprint and I used to flog myself out there. Every time. I would cycle home, 8km in the sun. I would get home and I would lay down in the shower. I would be sitting at the dinner table and I would be sitting there like this [slumps forward] ... I would be pale and mum would be going, 'you've just got to go easy, you know?'.
Lindsay played schools and junior rugby for Otago, and Robin played netball. Apart from the genetics, there was hard work and a singlemindedness that is almost second to none.
In May, 2012, seven months after winning the Rugby World Cup, Thorn won the Heineken Cup with Leinster, one of the Irish heavyweights. It came after 16 months of rugby without a break, including the traumatic 2011 season with the Crusaders.
He clearly likes his footy. His Heineken Cup achievement sits alongside the World Cup, eight tests for the Kangaroos league team, two State of Origin titles and several championships with the Brisbane Broncos, for whom he played 200 times.
Former all Blacks and Crusaders teammate Reuben Thorne says Thorn's one-track mind set him apart.
"He's just got a real competitive edge when he gets out on the field. You talk about guys who are ultimate professionals, well he's that guy who can flick that switch. Once he gets out on the field he turns into Brad the rugby player or Brad the league player, whatever it is," Thorne said.
"He shuts everything else out and focuses on his job. He goes from being a nice, friendly guy to being an aggressive, hard-nosed, ruthless rugby player and that's part of what being a professional is, you've got to have the ability to switch it on when you need to. He's one of those guys who can do that."
To learn about Thorn the rugby or league player, you have to learn about Thorn the man.
He is a person who values excellence above all else. Excellence in technique, of form.
He is always striving for improvement.
"I like doing stuff really well, whether that is making my bed or whatever. If I make my bed, it's crisp, it's tight, you know what I mean? I like striving for excellence so that drives me as well," he says.
"My dream is to always have immaculate technique. It's easy to have good technique, say in lineout lift five minutes into a game, but when there's five to go and you're tired and sweaty and whatever, that's the test."
As for his notorious feats in the gym, he says simply: "I love it in the gym, you're learning stuff all the time. Each week you go in and make gains. Sometimes I'll wake up at night and think about the next gym exercise I'm going to do, I'll be lying there and thinking about it."
Brad Thorn, one of a kind.
