Snakedance - S20-E2
Trivia: Brian Miller, who stars as Dugdale in this story, was the husband of Elisabeth Sladen, the actress who portrayed the Doctor's companion Sarah Jane Smith in the show from 1973 to 1976.
Trivia: The Doctor replaces the sprig of celery in his lapel with one conjured by the Eternals in this episode. The previous piece had been one he'd picked up in "Castrovalva", which was a Block Transfer construct created by Adric. This means that both pieces of celery were unreal constructs from artificial realities.
Trivia: Commander Maxil is portrayed by Colin Baker, who would later succeed Peter Davison in the role of the Doctor.
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