UKC

If replacement thyroid hormone treatment is assayed to be at reasonable levels, can the patient still be in need of lots of sleep? (My girlfriend seems to have a high requirement for long hours of sleep.)

Most definitely yes.

I  have hypothyroidism, rather than surgery to counter Grave's.
I am borderline over-medicated. 
It's where I function best.

I still fall asleep on the settee on a regular basis.
That's on top of 7-8 hr sleep at night.

Are there other reasons for sleepiness? Has this only happened since the surgery, or was it there before? Is there any snoring or stopping breathing overnight?

Worth discussing with her endocrinologist +/- GP

b

I do not know how things were before, as we have only been going out for a couple of months, but since the hyperthyroidism caused by Grave's disease is likely to reduce sleeping, I don't think so. 

Her work pattern is part of it, but her sleep response is greater than might be expected. She works two twelve hour night shifts on a Monday and Tuesday, and then a daytime nine to five shift on a Friday. I estimate that she sleeps a total of roughly 65 hours a week, possibly more. This is about twenty hours longer than me, and even though I know that I probably should sleep a little longer, it still seems excessive.

Possibly not excessive - about 9hrs / night is still well in the normal range for some people (including me these days). There has been some (not very good data) recently regarding 8hrs being optimal, but your 6hrs seems a little light... which again may be entirely normal for you, but just illustrating that there's quite a range of normality. If she has a lot of daytime sleepiness that might be different matter.

happy to discuss by pm if that helps

b