The human thyroid gland is a major component of the endocrine system. Thyroid hormones perform many important functions; they exert powerful and essential regulatory influences on growth, differentiation, cellular metabolism and general hormonal balance of the body, as well as on the maintenance of metabolic activities and the development of the skeletal and organ system. TSH stimulates the production and secretion of the metabolically active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T3 and T4 are responsible for regulating diverse biochemical processes throughout the body which are essential for normal development and metabolic and neural activity. Together, these three markers are important factors in the diagnosis of thyroid diseases, monitoring both patients under treatment for hyperthroidism and patients who have discontinued anti-thyroid drug therapy.