cat with overactive thyroid

[Soxy2154]

Our cat is 21 years old & costing a small fortune.
She lost a lot of weight & was constantly hungry so the vet did a blood test and said she has an over active thyroid, he gave us some medication which we had to give 2 times a day and to repeat the blood test in 2 weeks.
We got the result from the blood test yesterday & there has been no improvement in her thyroid score although she has put a kilo on.
He's now given us a different tablet which is to be given once a day and again to repeat the blood work in 3 weeks.
What I want to know is how quickly after starting theses meds should we see a difference? at the moment she is still constantly hungry and it's driving me mad as she constantly meows for food