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 Doctor

 * First Regeneration/Second Incarnation: Grew increasingly ill as Mondas drained his energy as well as Earth's, coupled with his body's advanced age.(DW: The Tenth Planet)
 * Second Regeneration/Third Incarnation: Enforced upon him by the Time Lords as part of his punishment for breaking the Laws of Time. (DW: The War Games)
 * Third Regeneration/Fourth incarnation: Caused by exposure to The Great One's highly unstable web of Metebelis crystals. (DW: Planet of the Spiders)
 * Alternate Third Regeneration/Fourth incarnation: Shot by Magdalena on the planet Dust after defeating the Remote. (EDA: Interference - Book Two) This timeline was created when the Eighth Doctor accidentally diverted his third self's TARDIS, and was later undone. (EDA: The Ancestor Cell).
 * Fourth Regeneration/Fifth incarnation: Unintentionaly caused by the Fourteenth Master by rotating the Pharos Project radio telescope to the point where he had a fatal fall to the ground. (DW: Logopolis)
 * Fifth Regeneration/Sixth Incarnation: Caused by contact with unrefined spectrox, causing a prolonged death, which he felt he would not be able to regenerate from. (DW: The Caves of Androzani)
 * Sixth Regeneration/Seventh incarnation: Caused by injury from the Rani's attack on the Doctor's TARDIS (DW: Time and the Rani). Accounts indicate the Doctor hit his head (NA: Head Games); however, he was also suffering from a chronal energy drain after his confrontation with the Lamprey (PDA: Spiral Scratch).
 * Seventh Regeneration/Eighth incarnation: Caused by gunshot wounds from the crossfire of an Earth gang war and exacerbated by hospital surgery which attempted to explore his unearthly physiology, but only succeeding in damaging his cardiovascular system far beyond repair. (DW: Doctor Who)
 * Eighth Regeneration/Ninth Incarnation: Unknown, but believed to have been caused by injuries sustained in the Last Great Time War. (IDW: The Forgotten)
 * Ninth Regeneration/Tenth incarnation: Caused by cellular degeneration from absorbing the energies of the time vortex from Rose Tyler to save her life and restore the TARDIS. (DW: The Parting of the Ways)
 * During tenth incarnation: Partial regeneration after being hit by Dalek fire; (DW: The Stolen Earth) aborted by channelling the energy into his severed hand, leading to the two-way biological metacrisis that would spawn the Meta-Crisis Tenth Doctor and the Doctor Donna (DW: Journey's End)
 * Tenth Regeneration/Eleventh Incarnation: Caused by severe radiation poisoning when the Doctor sacrificed himself to spare Wilfred Mott, absorbing all the radiation from the nuclear bolt used to power the Immortality Gate. It took a long time later rather instantly. (DW: The End of Time)

Alternate Doctors

 * The Doctor: Shot by Ruth after she learned the truth about his role in her father's death; she shot him again when he regenerated and so on. (BFDWU: Full Fathom Five)

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. (DW: Destiny of the Daleks)
 * Second to Third incarnation: Regenerated in preparation for the Second War in Heaven, developing a body more suited for combat. (EDA: The Shadows of Avalon)
 * Third to Fourth incarnation: Managed to survive the Second War in Heaven, although this may have entailed a regeneration (PDA: 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. (MA: The Dark Path)
 * During the Last Great Time War, the Time Lords resurrected the Master, apparently giving him a new regeneration cycle in the process (DW: The Sound of Drums) as, when the Master was shot by Chantho, he regenerated into a new incarnation. (DW: Utopia)

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

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

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

Rallon
Initiated all twelve of his regenerations to stop the Celestial Toymaker. (PDA: 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. (DW: The Twin Dilemma)

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

The War Chief
Underwent a faulty regeneration after being shot by War Lords. This resulted in his new form appearing like two bodies fused together. (NA: Timewyrm: Exodus)

Mortimus
Mortimus regenerated at least once. (NA: No Future, BFA: The Book of Kells)

I.M. Foreman
As a priest, he had been given the gift of regenerations. This made twelve different individuals, who were created by absorbing the DNA around him. (EDA: 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. (DW: 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. (DW: 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. (DW: Let's Kill Hitler)