Development of a radioimmunoassay for thyroxine in canines using polyethylene glycol

Metropolitan Clinical Laboratories, 687 West 8th Avenue, Vancouver, British Columbia, Canada Since the normal circulating level of thyroxine is so much lower in canines and other species than it is in humans, the authors suggest that it is impractical to use systems designed for thyroxine detection in human sera on their veterinary patients. Their newly developed, simple yet sensitive radioimmunoassay is presented here and further outlined in the accompanying table. Successfully performed on 500 canine patients, this radioimmunoassay features the use of T(4) standards prepared with T(4)-free serum, I(125) thyroxine with a very high specific activity, and specific antiserum for thyroxine, previously titered to give 50% binding of radioactive T(4) in the test system. Polyethylene glycol is used to separate the bound and free radioactive T(4), resulting in a procedure which can be performed rapidly at room temperature.