List of causes of regeneration

The causes of regeneration, especially amongst Time Lords, were many and varied. They ran the gamut from simple desire to change, to life-threatening maladies which typically resulted in human death. The following are the known causes of regeneration of specific Time Lords.

The Doctor

 * First Regeneration/Second Incarnation: Died of old age. (TV: The Tenth Planet)
 * During Second incarnation: Shot in the head when confronted by guards on Skybase, causing damage to his skull and frontal lobe; subsequent fall broke his nose, jaw, right femur, and collarbone, along with some spine damage; an injection of Shiner DNA delayed the regeneration and kept him alive long enough for his body to go into a six-month healing coma to recover on its own (PROSE: The Indestructible Man).
 * Second Regeneration/Third Incarnation: Shot by scarecrows animated by the Time Lords (COMIC: The Night Walkers), completing his sentence by the Time Lords. (TV: The War Games)
 * During Third incarnation: Fell off a building during a struggle, and began to regenerate into the Fourth Doctor as The Nurazh attempted to take over the Doctor's mind; unable to cope with two Time Lord minds at the same time, the Nurzah perished, thus healing the Doctor back to his Third incarnation. (PROSE: The Touch of the Nurazh)
 * Third Regeneration/Fourth incarnation: Caused by exposure to The Great One's highly unstable web of Metebelis crystals. (TV: Planet of the Spiders)
 * Alternate Third Regeneration/Fourth incarnation: Shot by Magdalena on the planet Dust after defeating the Remote. (PROSE: Interference - Book Two) This timeline was created when the Eighth Doctor accidentally diverted his third self's TARDIS, and was later undone. (PROSE: The Ancestor Cell).
 * Fourth Regeneration/Fifth incarnation: Caused when rotated the Pharos Project radio telescope so as to force the Fourth Doctor to fall to the ground. (TV: Logopolis)
 * Fifth Regeneration/Sixth Incarnation: Caused by contact with unrefined spectrox. (TV: The Caves of Androzani)
 * Sixth Regeneration/Seventh incarnation: Caused by injury from the Rani's attack on the TARDIS (TV: Time and the Rani). Accounts indicate the Doctor hit his head (PROSE: Head Games); however, he was also suffering from a chronal energy drain after his confrontation with the Lamprey (PROSE: Spiral Scratch).
 * Alternate Seventh Regeneration: In a reality where the Nazis used technology taken from Ace in Colditz to win the war, the Doctor arranged to be shot and killed in 1955 (AUDIO: Colditz); subsequently infiltrated the Reich as 'Johann Schmidt' to erase this new timeline (AUDIO: Klein's Story).
 * Seventh Regeneration/Eighth incarnation: Uniquely for the Doctor, this regeneration happened after his physical death. A gang shootout in San Francisco caused him near-fatal injuries, exacerbated by surgery at Walker General Hospital surgery which attempted to explore his unearthly physiology. The surgery only succeeded in damaging his cardiovascular system far beyond repair. He regenerated and came back to life in the morgue. (TV: Doctor Who)
 * During Eighth Incarnation: Stabbed in the heart, but healed by the energies of anti-time currently controlling his body (AUDIO: Zagreus); chest is crushed by a sandbag, but saved due to one heart having been transplanted into Sabbath, giving him a link to life that allowed his body to heal (PROSE: Camera Obscura)
 * Eighth Regeneration/Ninth Incarnation: Unknown. Believed to have been caused by injuries sustained in the Last Great Time War. (COMIC: The Forgotten)
 * Ninth Regeneration/Tenth incarnation: Caused by cellular degeneration from absorbing the energies of the time vortex from Rose Tyler in order save her life and restore the TARDIS. (TV: The Parting of the Ways)
 * During tenth incarnation: Partial regeneration after being hit by Dalek fire; (TV: The Stolen Earth) aborted by channelling the energy into his severed hand. This led to the two-way biological metacrisis that would spawn the Meta-Crisis Tenth Doctor and the Doctor Donna (TV: Journey's End)
 * Tenth Regeneration/Eleventh Incarnation: Caused by radiation poisoning when the Doctor sacrificed himself to spare Wilfred Mott. He absorbed all the radiation from the nuclear bolt used to power the Immortality Gate. Trivial cuts healed instantly. The regeneration, though, took a long time rather than instantly. (TV: The End of Time)

Romana

 * First to Second incarnation: Decided it was time for a change, regenerating into several different forms before settling on the features of Princess Astra. (TV: Destiny of the Daleks)
 * Second to Third incarnation: Regenerated in preparation for the Second War in Heaven, developing a body and attitude more suited for combat. (PROSE: The Shadows of Avalon)
 * Third to Fourth incarnation: Managed to survive the Second War in Heaven, although this may have entailed a regeneration. (PROSE: Tomb of Valdemar)

The Master

 * When his TARDIS was pulled into a Black hole by the Second Doctor, Koschei used up most his Regenerations escaping it. (PROSE: The Dark Path)
 * Shot by Ace after receiving a new set of regenerating nanites from the Tzun (PROSE: First Frontier) (Although these were apparently only effective in the short term).
 * During the Last Great Time War, the Time Lords resurrected the Master, apparently giving him a new regeneration cycle in the process (TV: The Sound of Drums) as, when the Master was shot by Chantho, he regenerated into a new incarnation. (TV: Utopia)

Rassilon
Rassilon was said to have achieved a cycle of perpetual regeneration, becoming immortal. (TV: The Five Doctors)

K'anpo Rimpoche
Attacked by the Eight Legs on Earth. (TV: Planet of the Spiders)

Borusa
Borusa regenerated at least three times, and had at least four lives. (TV: The Deadly Assassin, TV: The Invasion of Time, TV: Arc of Infinity, TV: The Five Doctors)

Chronotis
Salyavin used up his thirteenth and final life when he was attached by the Sphere. However, when his Type 12 TARDIS was sent backwards, it messed with his personal timeline and brought him back to life. (TV: Shada/WC: Shada)

Rallon
Initiated all twelve of his regenerations to force the Celestial Toymaker out of his body. (PROSE: Divided Loyalties)

Azmael
Azmael used all his twelve regenerations in unknown circumstances. In his Thirteenth and final body he regenerated past his limit, killing him and Mestor, who possessed Azmael's body after his own was destroyed. (TV: The Twin Dilemma)

Iris Wildthyme
Regenerated following a lengthy period of illness following the consumption of a live Kaled mutant. (PROSE: The Scarlet Empress)

The War Chief
Underwent a faulty regeneration after being shot by War Lords; lack of medical care and the scale of the damage sustained resulted in his new form appearing like two bodies fused together. (PROSE: Timewyrm: Exodus)

The Monk
Mortimus regenerated at least once. (PROSE: No Future, AUDIO: The Book of Kells)

Ruath
Drained every drop of her blood from her body to restore Vampire Lord Yarven (Yarven subsequently turned her into a vampire) (PROSE: Goth Opera).

Innocet
Killed by a Quences-possessed Badger to protect the Doctor (PROSE: Lungbarrow).

I.M. Foreman
As a priest, he had been given the gift of regenerations. This made twelve different individuals, who were created by his body absorbing the DNA around him; all regenerations were caused by the Third Doctor sending the first twelve back to Gallifrey's past so that they fell from a great height and regenerated into the next one. (PROSE: Interference - Book One, Interference - Book Two)

The Corsair
The Corsair regenerated several times, with "a couple" of his incarnations being female. He was back to male by the time he was killed by House. (TV: The Doctor's Wife)

River Song

 * First to second incarnation: As a little girl, Melody Pond, later known as River Song, regenerated on the streets of New York City in 1970 due to a terminal illness. (TV: Day of the Moon)
 * Second to third incarnation: Melody later regenerated in Hitler's private study in 1938 after the Führer accidentally shot her while fearing the Teselecta. She later used up all of her remaining regenerations to heal the Doctor from a poison that she had infected him with. (TV: Let's Kill Hitler)