I am sorry Maisie is unwell.
Hyperthyroidism is very common in cats and is usually easy to manage. It is good that she has a follow up appointment tomorrow as the bloods will show whether the medication is working and if the correct dose is being used.
Regarding the lethargy today, it is a tricky one as it may well be she has picked up something completely unrelated to the hyperthyroidism. IF she is very quiet, lethargic and not eating and most importantly not drinking then you should consider getting her checked over today. It is vital for cats to remain hydrated and being older, she may not tolerate prolonged periods of starvation. Possible problems include pain from an injury or cat bite to developing gastroenteritis or other organ disease.
If she is alert, happy and bright and is drinking then you could consider watching and waiting through the day and if stable getting a thorugh exmaination tomorrow, but if deterioartion is seen through the day then a trip to the out of hours vets would be beneficial, even it is just to give you peace of mind.
Good luck and I hope she is OK