Abnormal circulating T4 concentration 2017-09-21T01:52:52Z A deviation from 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). Abnormal circulating thyroxine level Abnormal circulating T4 level Abnormal circulating thyroid hormone concentration