In excess, it both overstimulates metabolism and disrupts the normal functioning of sympathetic nervous system, causing "speeding up" of various body systems and symptoms resembling an overdose of epinephrine (adrenaline). These include fast heartbeat and symptoms of palpitations, nervous system tremor such as of the hands and anxiety symptoms, digestive system hypermotility, unintended weight loss, and, in lipid panel blood tests, a lower and sometimes unusually low serum cholesterol.Major clinical signs of hyperthyroidism include weight loss (often accompanied by an increased appetite), anxiety, heat intolerance, hair loss (especially of the outer third of the eyebrows), muscle aches, weakness, fatigue, hyperactivity, irritability, high blood sugar, excessive urination, excessive thirst, delirium, tremor, pretibial myxedema (in Graves disease), emotional lability, and sweating. Panic attacks, inability to concentrate, and memory problems may also occur. Psychosis and paranoia, common during thyroid storm, are rare with milder hyperthyroidism. Many persons will experience complete remission of symptoms 1 to 2 months after a euthyroid state is obtained, with a marked reduction in anxiety, sense of exhaustion, irritability, and depression. Some individuals may have an increased rate of anxiety or persistence of affective and cognitive symptoms for several months to up to 10 years after a euthyroid state is established. In addition, those with hyperthyroidism may present with a variety of physical symptoms such as palpitations and abnormal heart rhythms (the notable ones being atrial fibrillation), shortness of breath (dyspnea), loss of libido, amenorrhea, nausea, vomiting, diarrhea, gynecomastia and feminization. Long term untreated hyperthyroidism can lead to osteoporosis.
There is also a special low iodine diet available that will control the symptoms providing no other food is fed; Hills y/d formula, when given exclusively, decreases T4 production by limiting the amount of iodine needed for thyroid hormone production. It is the only available commercial diet that focuses on managing feline hyperthyroidism. Medical and dietary management using methimazole and Hills y/d cat food will give hyperthyroid cats an average of 2 years before dying due to secondary conditions such as heart and kidney failure. Drugs used to help manage the symptoms of hyperthyroidism are methimazole and carbimazole. Drug therapy is the least expensive option, even though the drug must be administered daily for the remainder of the cats life. Carbimazole is only available as a once daily tablet. Methimazole is available as an oral solution, a tablet, and compounded as a topical gel that is applied using a finger cot to the hairless skin inside a cats ear. Many cat owners find this gel a good option for cats that dont like being given pills. Radioiodine treatment, however, is not available in all areas, as this treatment requires nuclear radiological expertise and facilities that not only board the cat, but are specially equipped to manage the cats urine, sweat, saliva, and stool, which are radioactive for several days after the treatment, usually for a total of 3 weeks (the cat spends the first week in total isolation and the next two weeks in close confinement).