Thyroxine is a hormone secreted by the thyroid gland that responds to the function of the thyroid gland and includes free thyroxine and bound thyroxine in the blood. The proportion of bound thyroxine is relatively large, while the proportion of free thyroxine is relatively small. Although the proportion is smaller, free thyroxine is biologically active thyroxine. It is a good indicator of thyroid activity because it is not affected by the concentration of binding proteins. In addition to free thyroxine, thyroid stimulating hormone and triiodothyronine are clinically indicated. An increase in free thyroxine and a simultaneous decrease in thyroid stimulating hormone suggest a high probability of hyperthyroidism, and it is best to perform a thyroid iodine uptake test at the same time. In addition to hyperthyroidism, the early stages of subacute thyroiditis may also show elevated free thyroxine.