Trivia: This serial was the first to feature the Doctor's favourite gadget, the sonic screwdriver. The sonic screwdriver has popped up in Doctor Who stories many times since, all the way into the new series. Sadly, it cannot be seen in "Fury from the Deep" as every episode of that story was destroyed by the BBC in 1977.
Trivia: Only episode one of this six part Doctor Who story survives, but it has been digitally restored by the BBC to 'broadcast quality'. Indeed, it WAS broadcast on the BBC's digital channel BBC4 as recently as June 26, 2004, as part of that channel's Sixties Season.
The Enemy of the World - S5-E4
Trivia: This serial was believed lost until 2013, when all of the episodes were discovered and returned to the BBC. It was subsequently restored and released on DVD.
Chosen answer: No, it was the result of the Cybermen's draining Earth of energy to their world, that also affected the Doctor, draining his energy as well, causing him to regenerate.
Also In the episode it's said that his body was wearing thin.
Dan23