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Hi. Welcome to Just Answer. My name is***** and I've been a veterinarian for 15 years. Thank you for your question about Mona. I'm sorry to hear about her urinary accidents. I wouldn't expect these accidents to be due to her hyperthyroid situation or due to the medication she is on. With her inappropriately urinating, this could be a behavioral issue or a physical issue. The only way to know if this is a physical issue is to have her seen again by your vet to have a urine sample evaluated. Inappropriate urination like this could be due to a urinary tract infection that needs to be treated with a course of antibiotics. If it isn't a urinary tract infection, then could it be a behavioral issue where she is doing this because of some "stressor". Here is a good link on feline house soiling due to behavioral issues and what can be done to help that.
As far as should you be concerned, I would be concerned enough to get her seen to see if she has the urinary tract infection as that needs specific therapy for it. Please let me know what questions this brings up.