I've had cats ever since I left home (40 years ago), and my little 18yr old cat was put to sleep a year ago. She, too, had been on medication for an overactive thyroid and kidney failure, and she went rapidly downhill just the day before we had to have her put to sleep. She was very vocal, and I found the house really quiet (despite having another cat, who now loves being the only one).
Your little cat, like mine, had a long and happy life. You loved her, cared for her, and enjoyed having her around - but she was old and unwell. It was her time to go.
You could think about having another cat, from a shelter, perhaps (all of mine were rescue cats, and my boy now was a stray). Give yourself time, though, and don't rush into getting a new cat yet. x
Humza Yusaf's resignation speech