I'm only going on personal experience, and experience of people I know, (no medical qualification whatsoever), BUT
Hope you are being a bit jokey when you say the Doctor says there's nothing wrong with her and she can at least say that she has had relevant blood tests to establish whether her Thyroxine levels are borderline or whether, in fact, she might already have Hypothyroidism - if that's your worry. If her blood results show that's not the problem and she's not very well, she ought ask for a referral to an Endocrinologist for further investigation - a sort of health MOT ! A GP can only know so much - so someone who specialises in hormonal problems is likely to have different ideas and probably offer much more help.
If the GP is happy to refer, from what I recall when I lived in the UK, he'll write to one for an appointment through the NHS. In the unlikely event that he refuses - because he thinks he knows the answer to all things/or he is adamant that your Daughter is fit and well, then she would possibly do better to consider a private consultation. This usually requires a letter of referral, but checking with the Endo's private Secretary would confirm this and also let you know the cost of a consultation before you made up your mind whether to go down that route. I think that the worst thing is doing nothing when you feel that your health is just not right. Carrying on putting up with stuff - isn't that what a lot of us women do? A diagnosis is most important - feeling that you are being a nuisance or being dismissed because you have vague problems which you are putting up with, while not having much hope of feeling better...and soon...is not really the way to go. Just one life we have and it's been pretty dire over the last long months - so getting out there and attempting to get ourselves well is probably a really good idea! Good luck.