T4 (Thyroxine) is one of the major hormones produced by the thyroid gland, and exists in protein-binding (bonded to thyroxine globulin, albumin, etc.) and free-form (free T4) in the body. If the thyroid gland does not produce enough T4, symptoms of hypothyroidism such as weight gain, dry skin, cold sensitivity, irregular menstruation and fatigue will occur, and if the thyroid gland produces excess T4, the speed of functions in the body will increase, which will lead to hyperthyroidism-related symptoms such as increased heart rate, anxiety, weight loss, insomnia, hand tremors, and swelling around the eyes.