Nastycat had treatment with radioiodine a couple of years ago for an overactive thyroid. She's never been a pleasant cat to live with but she is currently driving us up the wall. She sleeps very little (especially compared to our two year old who is fast asleep all day and evening, occasionally getting up to eat - he is out all night getting wet and muddy and hunting). She howls non-stop all day and much of the night. Her fur is scraggly looking. She is clingy with me but aggressive with everyone and everything else. This all sounds like thyroid, but that is meant to have been cured by the very expensive treatment (and she had tests a year after to check it had worked). Can it come back? She had a good check up last year during her inappropriate peeing episode and the vet thought the howling, which was nowhere near as bad than, was down to her age.