Thyrotoxicosis is the term given to any condition that causes exposure of the tissues to high serum concentrations of free thyroxine (T4), free triiodothyronine (T3), or both that results with hypermetabolism and hyperactivity. Hyperthyroidism is the term given to the subgroup of diseases in which the reason of thyrotoxicosis is increased thyroid hormone synthesis and secretion by the thyroid gland.