Venous sampling is a medical procedure where blood is drawn from a vein that drains from a particular organ or gland and then is tested for the typical products or hormones that are produced by that organ or gland that are typically released into the bloodstream. Unlike a regular blood test from a vein, which is minimally invasive, venous sampling is very invasive and usually requires some form of radiology to guide the physician performing the procedure.
It is very commonly used to test whether deficiencies in hormones produced by two glands (such as the adrenal glands) is unilateral (a problem with one of the glands) or bilateral (a problem with both glands). It is also commonly used to test the thyroid gland.