Weight loss, increased water consumption and urination are the most common signs of feline hyperthyroidism, or overactive thyroid gland, states Cornell Feline Health Center. Hyperthyroid cats may also vomit and have loose stool. Symptoms of hypothryoidism, rare in cats, include depression, hair loss, sluggishness and weight gain, notes petMD.
The thyroid hormone controls metabolism, so many of the symptoms of feline hyperthyroidism relate to an increase in metabolic rate, according to the Merck Veterinary Manual. During a physical examination, a veterinarian can detect more subtle signs, such as irregularities in heartbeat or enlargement of the thyroid gland. Blood testing is necessary to confirm the condition.