Retinoid refers to retinol-binding protein. It is a transport protein for vitamins in the blood, synthesized in the liver, and present in a variety of body fluids, including blood and urine. Retinol-binding protein levels, as a result of some diseases, are used to help doctors differentiate between diseases. For example, a decrease in retinol-binding protein is a sensitive indicator of liver damage and malnutrition, and its elevation is a sensitive indicator of impaired glomerular filtration. Levels in urine are highly correlated with renal tubular reabsorption function. In addition, insulin resistance, a biomarker of diabetes and cardiovascular biomarkers, correlates with elevated serum retinol-binding protein.