Feline hyperthyroidism is caused by the overproduction of hormones from a cat’s thyroid gland. Thyroid glands are a pair of butterfly-shaped glands located on either side of the windpipe. This affliction is one of the most common hormonal disorders in middle-aged and senior cats. Hyperthyroidism is usually caused by a benign tumor that produces excess thyroid hormones on one or both thyroid glands. Most cases of hyperthyroidism are mild, with many cats not showing any signs of the illness.