Radionuclides can be administered in very small doses orally or IV and, at a later time, detected in the body by laboratory testing of blood, urine, and other body fluids. These tests determine the ability of the body to absorb the radionuclide by measurement of the concentration of radioactivity. Glands such as the thyroid can also be examined for the concentration of radioactivity by determining the ability of the body to localize the radionuclide as in the use of iodine radionuclide in thyroid testing. Radionuclides and laboratory tests are listed in Table 20-1. Scanning is performed in combination with laboratory testing for some studies.¹¹ Depending on the test to be conducted, the laboratory, the department of nuclear medicine, and the client or the hospital staff share responsibility in performing these studies.

A wide variety of substances can be measured by the radioimmunoassay studies included in the laboratory section of this book. They include hormones, proteins, immunoglobulins, carcinogens, antibodies, vitamins, and drugs that are antigenic or that can be made so by adding them to an antigenic substance. These tests involve the administration of a labeled compound that is produced by chemically binding the radionuclide to another molecule. They can be labeled to antigens or antibodies because the assay is based on an antigen-antibody reaction to detect and measure a substance. This method of testing provides a high level of sensitivity and specificity in substance detection. Easily measured isotopes such as iodine 25 (25I), carbon 14 (14C), or cobalt 57 (57Co) are labeled and used in this method. Some substances are measured in the blood by the tagging of proteins with radionuclides. Protein-binding properties are the basis for estimating the amount of substances such as cortisol, folate, thyroxine, and vitamin B12 because the radionuclide-labeled compound competes with existing compounds for these binding sites. This competition permits the measurement of the substances that are bound to the protein, even in minute amounts.^12,13