The diagnosis and treatment of endocrine diseases are guided by laboratory tests to a greater extent than for many specialties. Many diseases are investigated through excitation/stimulation or inhibition/suppression testing. This might involve injection with an interesting agent to check the function of an endocrine organ. Blood is then sampled to assess the changes of the relevant hormones or metabolites. An endocrinologist needs extensive knowledge of clinical chemistry and biochemistry to know the uses and limitations of the investigations.