Abnormal circulating T4 concentration Decreased circulating T4 concentration 2017-09-21T01:55:27Z Decreased circulating T4 level A reduction below 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). Decreased circulating thyroxine level Reduced T4 plasma level