Para thyroid, help!

I have a tricky case involving a client with an overactive parathyroid and an Employment and Support Allowance appeal. Essentially it seems as though it will come down to a question of reasonable regularity in the context of the limited capability for work assessment. On the ESA50 the client has indicated no problems save for qualifying this with the comments if I stand, walk etc for too long it makes me very fatigued. 

I was also thinking of the exceptional case regulation as obviously if she is working then there is a risk to her health and indeed it was the basis of her claim to Employment and Support Allowance as the client kept telling herself she was ok and then collapsed followed by a period in hospital.

Further the client is due to have an operation to remove the parathyroid and will from then on be prescribed calcium supplements etc

Anyone every come across this condition before in relation to Incapacity Benefit or Employment and Support Allowance appeals?

Thanks in advance.