Organ function tests are the biochemical tests carried out to assess whether particular organ is functioning normally or not. This chapter describes liver function tests, renal function tests and thyroid function tests. Unconjugated hyperbilirubinemia with normal alanine transaminase (ALT) activity indicates the presence of haemolytic jaundice. Conjugated hyperbilirubinemia with raised activity of the ALT indicates hepatic jaundice; however, conjugated hyperbilirubinemia with marked elevation of alkaline phosphatase (ALP) activity suggest obstructive jaundice. The glomerular filtration rate (GFR) is estimated by measuring creatinine clearance. Assessment of concentrating and diluting ability of the kidney provide most sensitive means of detecting early impairment in renal function. Urine analysis reveals the disease anywhere in the urinary tract. The test used to investigate thyroid dysfunction can be performed in vivo or in vitro. Serum T3, T4 and TSH help to assess the severity of thyroid disease.