now on @villiarty

You’re not allowed to do that. So, many, many years later they found a way to retcon it, as they say, but it was just… It’s interesting. The audience is in control of what sticks. James Bond’s scar? Nope. Doctor Who’s real name? No. Sherlock Holmes would never wear a deerstalker in the middle of town — yes, he would. Because we like him in that.

Here’s the line to which I’m assuming he’s referring, from the Happiness Patrol (season 25 ep 2):

I don’t know about you, but my real name isn’t something that I mention as an aside while specifying it was my old school nickname. Lots of people consider a specific nickname to be their real name, but I think the context here is sufficient to assume that it’s not the case for the doctor.

So, I’m not sure why Moffat felt the need to emphasize (three times!) that this was the doctor’s “real name” but it’s not. Yes, this was their nickname at (presumably) the academy, no it is not a name they currently use, have used for any significant length of time, or consider their name.

Hope this clears things up!

(also i added alt text but I’ll put an id under the cut in case it doesn’t work)

Happiness Patrol aired 1988: 25 years after the showed started, made by a large number of people who’d grown up watching the show and had opinions on things like “we shouldn’t know his name”.

But “Theta Sigma” is first in Armagggedon Factor: 1979.  A decade earlier, a completely different production team. And in that episode, it’s—

 DRAX: It is Theet, innit? Theta Sigma? Yeah, ‘course it is. Remember me, ay?

The nickname in that episode is “Thete” - there’s no suggestion that Theta Sigma is anything but the Doctor’s actual standard full name, as attested by a guy who was in the same class as him 450 years ago. (And got stuck in Brixton for a while, hence the speech patterns.)