Free thyroxine is secreted by the thyroid gland and is a major part of the biological effect of the hormone. Therefore, high levels of free thyroid hormone are usually caused by thyroid disorders such as hyperthyroidism, thyroiditis, hypothyroidism, and chronic thyroiditis. When the thyroid tissue is attacked by lymphocytes, the thyroid cells are destroyed and a large amount of thyroxine is released into the bloodstream, which causes the symptoms of hyperthyroidism. Free thyroxine is derived from the shedding of thyroglobulin, which plays a major role in promoting thyroid metabolism in the body. High serum free thyroxine is commonly seen in two clinical conditions: hyperthyroidism and subacute thyroiditis, both of which have high free thyroxine, but there is a difference between these two conditions. Therefore, when a physical examination reveals elevated free thyroxine in the serum, the first thing to consider is that it is caused by hyperthyroidism or thyroiditis. Ultrasound of the thyroid gland and thyroid function tests will be needed to confirm the diagnosis. It is important to develop good habits in daily life, eat more food with high iodine content, and strengthen physical exercise to improve your body’s resistance and immunity.