...
There have been a few Time Lords who were ladies. Two, I think.
Time Lady
Romana, played by two different actresses, and The Master as Missy, in the recent series are the most obvious two.
There was also the Corsair, who has also been referred to multiple times as being both male and female, although (s)he doesn't make an appearance (possibly excepting Auntie's forearm in The Doctor's Wife, which is alluded to as being the Corsair's).
A number of unnamed Time Ladies also appear, in various episodes.
Googling also says:
Rodan,
Darkel, and
The Rani.
... oh, and the Doctor himself has been female before, during Comic Relief, portrayed by
Joanna Lumley.
... oh, oh, oh, do Doctor Donna or River Song count? They're both part Time Lord (Lady), aren't they?