Your are describing the symptoms of this to a "T". Our cat, who has recently been diagnosed hyperthyroid was identical. Get your cat to a vet and have her blood tested. They can check for kidney, liver and thyroid levels.
Our cat was lethargic, however, so sometimes not all symptoms apply. However, being ravenous and yet still not eating is classic. Some sites describe it as "feline anorexia". Basically what we learned is that when the thyroid is off it affects the taste of the food to your cat so she won't eat. Also, even if the cat does eat it won't gain weight (loses instead) due to the overactive thyroid.
Check it out and if your cat is hyperthyroid maybe we can share notes and experiences and help each other out. My cat is on a transdermal gel medication, not a pill. You put in on the skin on the inside of the ear. Much easier than pilling and it doesn't have the side effects to the gi tract.
Let me know how it goes.
BTW, ZonkaDaisy, hair falling out can also be a symptom of hyperthyroid. Just an FYI.