T4 (Thyroxine) is the main hormone produced by the thyroid gland. T4 is not biologically active, and must be converted to T3 before it can become active in regulating metabolism. Some T4 is bound to proteins, while some T4 is free or unbound. This test measures the level of total T4 in your blood.
What might a low result mean?
Low levels of T4 may indicate dietary issues such as or an iodine deficiency, hypothyroidism or a pituitary problem.
What might a high result mean?
High T4 levels may indicate hyperthyroidism, or other thyroid problems, such as thyroiditis. High levels of protein in the blood and too much iodine in the body can also cause elevated T4 levels.