The Regeneration of Dr Who

So far, the Doctor is not immortal.  The changes come, but they are tempered by their limited number.  He's more like the proverbial cat with nine lives rather than truly living forever.

As for the things which kill him, they are pretty much what might kill human beings too.  Though granted, we don't tend to find ourselves in most of these situations.

• The First Doctor (William Hartnell) died of debilitation in old age.  He'd been fighting Cyberman in the Antarctic, when the strain grew too much.  He collapsed in the TARDIS. His horrified companions, Ben and Polly, witnessed his renewal.
• The Second Doctor (Patrick Troughton) had no choice in his regeneration. He stood accused of interference in a court assembled by his fellow Time Lords.  His punishment was to be exiled to Earth, without any knowledge of his TARDIS. His companions, Jamie and Zoe, were returned to their own point of origin, similarly stripped of their memories of The Doctor. By the time the 2nd Doctor crash-landed onto Earth, the trauma was enough to force his body to change.
• The Third Doctor (Jon Pertwee) was fatally exposed to radiation from crystals, while facing down giant spiders on the planet of Metebelis III.  Sarah Jane and the Brigadier were there as he died, then stood shocked by the sudden appearance of K'anpo Rimpoche. The genie-like avatar was also a Time Lord and the Doctor's old mentor. K'anpo intervened to trigger the Doctor's regeneration.
• The Fourth Doctor (Tom Baker) fell from an upturned radio dish, which the Master was rotating in a successful attempt to dislodge him. As Nyssa, Tegan and Adric raced to the Doctor's side, the shrouded figure of the Watcher was spotted waiting by.  Though suffering multiple injuries, the broken Doctor summoned the strength to wave the Watcher closer. It stepped into him and the regeneration process occurred.
• The Fifth Doctor (Peter Davison) was gassed to death on the planet Androzani Minor. He had the antidote to the deadly toxin Spectrox, but only one dose.  His companion Peri was also dying, so the Doctor heroically staved off his own regeneration. He carried her to the TARDIS and gave her the antidote, before collapsing himself.  He'd previously worried that he wouldn't be able to renew himself, as he'd waited too long while dying. The terrified Peri cowered in horror, as the Doctor transformed anyway.

• The Sixth Doctor (Colin Baker) was fatally injured/forced into a regeneration during an attack on the TARDIS by a Time Lord named The Rani.  It was never actually explained on-screen what triggered it.  All viewers (and companion Mel) saw was the Doctor prone on the floor of the TARDIS. Unconscious, he was turned onto his back and revealed as having regenerated.  It's a plot hole beloved of Whovian fan fiction writers.
• The Seventh Doctor (Sylvester McCoy) was shot by gangsters in San Francisco. This shouldn't have been fatal, but medics at the local hospital had no idea they were operating on an alien. Surgeons heard the dual beating of two hearts and assumed it was a single one in need of fibrillation. The shock killed him. However, anesthesia delayed the onset of his regeneration. He later changed, all alone, in the hospital morgue.
• The Eighth Doctor (Paul McGann) crash-landed on the Planet Karn. He was offered the choice to control his regeneration with a draft prepared by the Sisterhood of Karn. Furious with the state of the war between Time Lords and the Daleks, he chose to become the Warrior.
• The Warrior (John Hurt) will be shown in the 50th Anniversary Special.
• The Tenth AND Eleventh Doctor (Christopher Eccleston) effectively commits suicide with a kiss.  His beloved companion Rose has looked into the heart of the TARDIS, thus absorbing into herself time vortex energy.  It's all too much for her fragile human form to withstand, so the Doctor draws it into himself.  Nor could he safely absorb it, but at least he could regenerate, while Rose looked tearfully on.
• The Twelfth Doctor (David Tennant) is poisoned by radiation, while rescuing Wilfred Mott from the Gate to a Time Lock.  His death is perhaps the most poignant of all the regenerations thus far, because of its longevity and the fact that this Doctor didn't want to die.  He spends hours secretly visiting all of his past companions or their descendants, though he doesn't reveal himself to all. Ultimately he dies alone on the TARDIS; while out in the universe, the Ood sing to his passing, 'This song is ending, but the story never ends.'
• The Thirteenth Doctor (Matt Smith) died of old age and being shot at by Daleks. Tellingly, it was the first item which was the pertinent point. We can't have the Doctor dying to a mere Dalek!

Peter Capaldi starts a whole new regeneration cycle of thirteen lives, which technically makes him the Second First Doctor.  But that's going to be way too confusing.  The likelihood is that we'll all forget that The Warrior was the Ninth, and pin that accolade back onto Christopher Eccleston. Then ignore too his double regeneration, to make David the tenth and Matt the eleventh.

Peter will sail into the Whovian hive-mind as the twelfth Doctor. Even if he is the Second First.

Nevertheless, he's still with us and therefore not included in this round up.

Was it sarcasm?  A throw-away line designed to impress a teenager?  Whatever his motives, the Eleventh Doctor ran contrary to all previously established facts, when he informed Clyde that there were 507 regenerations in him.

In truth, Whovians had been expecting it.  After all, the BBC were not going to let such a lucrative franchise die, just because none of their Doctors could apparently survive longer than eight years in a single form.  Something was going to have to change. Whole forum threads and conference debates were staged to speculate on how Time Lord lore would be changed.

The reality was a bit of an anti-climax.  It didn't even occur in a Dr Who story, but in a spin-off - The Sarah Jane Adventures.  From 12 to 507 wasn't a bad difference.  It would take a lot of regenerations before they were all worked through. But was it a cop-out?

Director Russell T Davies stated that it was just too good an opportunity to miss. Fans just face-palmed. Many refused to even consider it canon, as it hadn't been stated in the main show. Everyone wanted it, but not like this.  There had to be some great story device to change things, not the laws of Time Lord biology simply being erased with a word.

However, it all transpired to be a joke. The Doctor did only have thirteen regenerations in him, unless Gallifrey saw fit to award him another cycle.  And guess what they did!  Who would have thought it?