Updated March 12, 2021
T4 is the main hormone made by the thyroid gland. It is also the main medication used by physicians to treat low thyroid disorders, T4, which as a pharmaceutical is called Synthroid, Levothroid, Levoxyl, or the generic thyroxine.
T4 is not the active hormone. It is activated by many cells of the body (liver, muscle, heart, brain, immune) to T3, also called tri-iodo-thyronine.
Many people have difficulties converting T4 to T3 for many reasons which will be outlined elsewhere. Therefore, even if the T4 and TSH levels are “within normal limits”, they will have symptoms of hypothyroidism, even if mild…