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On screen in "The Tenth Planet" the exact reasons aren't spelled out beyond the Doctor commenting on his body "wearing a bit thin" and being in generally poor shape throughout the story. Generally speaking "old age" (at least 450 years if the number given by the Second Doctor in "The Tomb of the Cybermen" is accurate), coupled with the energy draining scheme of the Cybermen are cited as the triggers that caused his first regeneration. Some writers examining the history of the series have suggested the First Doctor never recovered from the events of "The Daleks' Master Plan" and the stories that followed were the lead up to his eventual regeneration.