The concentration of hormone present in a biological fluid is often measured to make a clinical diagnosis of a suspected endocrine disease or to study basic endocrine physiology. Substantial advancements have been made in measuring hormone concentrations.
Bioassay. Even before hormones were chemically characterized, they were quantitated in terms of biological responses they produced. Thus, early assays for measuring hormones were bioassays that depended on a hormone's ability to produce a characteristic biological response. As a result, hormones came to be quantitated in terms of units, defined as an amount sufficient to produce a response of specified magnitude under a defined set of conditions. A unit of hormone is, thus, arbitrarily determined. Although bioassays are rarely used today for diagnostic purposes, many hormones are still standardized in terms of biological activity units. For example, commercial insulin is still sold and dispensed based on the number of units in a particular preparation, rather than by the weight or the number of moles of insulin.
Bioassays in general suffer from a number of shortcomings, including a relative lack of specificity and a lack of sensitivity. In many cases, they are slow and cumbersome to perform, and often they are expensive, since biological variability often requires the inclusion of many animals in the assay.
Radioimmunoassay. Development of the radioimmunoassay (RIA) in the late 1950s and early 1960s was a major step forward in clinical and research endocrinology. Much of our current knowledge of endocrinology is based on this method. A RIA or closely related assay is now available for virtually every known hormone. In addition, RIAs have been developed to measure circulating concentrations of a variety of other biologically relevant proteins, drugs, and vitamins.
The RIA is a prototype for a larger group of assays termed competitive binding assays. These are modifica
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