Abnormal circulating T4 concentration Increased circulating T4 concentration 2017-09-21T01:54:51Z Increased circulating thyroxine level An elevation above the normal concentration of thyroxine in the blood. Thyroxine (also known as T4) is the main hormone secreted by the thyroid gland into the blood. It can be converted into the active form triiodothyronine (also known as T3). Increased circulating T4 level