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In a 14 year old cat, hyperthyroidism can be very common. It very commonly does have quite severe side effects on the heart, and most hyperthyroid cats have some degree of cardiomyopathy. Most cases of cardiomyopathy in cats are of the Hypertrophic type which can preclude them to throwing clots. Thromboembolic events or clots can cause various problems in the body ranging from weakness to paralyis to obstruction of cerebral vasculature which could manifest as a stroke or in rare instances possibly induce a seizure.
So to answer your question, hyperthyroidism does not directly cause seizures in the truest sense. Instead, cats with cardiomyopathy secondary to hyperthyroidism can often develop hypertension ( high blood pressure) which can result in vascular accidents or hypoxia( oxygen loss at the tissue level) which may cause seizures.
I would strongly recommend thoracic radiographs, an EKG,and several periodic blood pressure measurements over the course of the day. Your pet should commence treatment of the hyperthyroidism with Methemizole as directed by your veterinarian and I would also consider the usage of Enacard and Baby Aspirin therapy to treat the underlying cardiomyopathy.
Your vet will need to do serial bloodwork testing over the course of Methimazol (Tapazole) treatment in order to insure that the proper maintenance dose is established and that your cat's thyroid levels are back to a normal physiologic level.
Your pet's hyperthyroidism may be due to an actual tumor of one of the thyroid glands ,so surgical removal of the affected gland or radiation therapy may be called for in place of Methamizole therapy.
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Best Wishes to You and Your Pet! Dr. Jodi L. Smith