I've just said that I have a Dr.'s appointment. Or I've just said I have an appointment. Not really any of their business what the appointment is.
I've been lucky, though, that neither of the T's I've seen have me on a particular schedule. I go typically once every 2 weeks and the time and day varies. Makes it easier on me at work since I can sometimes schedule it for the end of the day.
I have had appointments around lunch time and just said that I have an appointment. That's it. It didn't need to go any further than that.