Bitiron is a synthetically derived thyroid hormone replacement preparation. It consists of levothyroxine sodium (thyroxine, T4) and liothyronine sodium (triiodothyronine, T3) in a 4 to 1 ratio by weight. Liotrix was developed when it was believed that serum levels of both T4 and T3 were maintained by direct thyroidal secretion. It is now known that the thyroid gland secretes approximately ten times more T4 than T3 and that 80% of serum T3 is derived from deiodination of T4 in peripheral tissues. Administration of levothyroxine alone is sufficient for maintaining serum T4 and T3 levels in most patients and combination hormone replacement therapy generally offers no therapeutic advantage. In fact, administration of T3 may result in supratherapeutic levels of T3.
Thyroid hormones are essential for proper development and differentiation of all cells of the human body. T4 and T3 regulate protein, fat and carbohydrate metabolism to varying extents. The most pronounced effect of the hormones is in altering how human cells use energetic compounds.