High free thyroxine, usually due to hyperthyroidism, causes hyper-metabolism, mental excitement, palpitations, sweating, increased appetite, weight loss, more frequent bowel movements, insomnia and dreaminess, and hand tremors. Some may also have protruding eyes. Diffuse toxic goiter, subacute thyroiditis, Hashimoto’s disease, some drug stimulation, pituitary tumors can cause hyperthyroidism with high free thyroxine. Hyperthyroidism needs to be treated promptly, otherwise it can lead to serious complications such as hyperthyroid heart disease and heart failure.