Porth's Essentials of Pathophysiology, 4e

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Endocrine System

U N I T 9

Real progress in measuring plasma hormone levels came more than 40 years ago with the use of competi- tive binding and the development of radioimmunoassay (RIA) methods. This method uses a radiolabeled form of the hormone and a hormone antibody that has been prepared by injecting an appropriate animal with a puri- fied form of the hormone. The unlabeled hormone in the sample being tested competes with the radiolabeled hormone for attachment to the binding sites of the anti- body. Measurement of the radiolabeled hormone–anti- body complex then provides a means of arriving at a measure of the hormone level in the sample. Because hormone binding is competitive, the amount of radio- labeled hormone–antibody complex that is formed decreases as the amount of unlabeled hormone in the sample is increased. Newer techniques of RIA have been introduced, including the immunoradiometric assay (IRMA). IRMA uses two antibodies instead of one. These two antibodies are directed against two different parts of the molecule, and therefore IRMA assays are more specific. RIA has several disadvantages, including limited shelf life of the radiolabeled hormone and the cost for the disposal of radioactive waste. Nonradiolabeled methods have been developed in which the antigen of the hormone being measured is linked to an enzyme-activated label (e.g., fluorescent label, chemiluminescent label) or latex particles that can be agglutinated with an antigen and measured. The enzyme-linked immunosorbent assays (ELISAs) use antibody-coated plates and an enzyme-labeled reporter antibody. Binding of the hormone to the enzyme-labeled reporter antibody produces a colored reaction that can be measured using a spectrophotometer. In many situations, immunoassays are unreliable or unavailable. For some steroid or peptide hormones, mass spectrometry is becoming increasingly useful and can be combined with other analytical techniques, such as liquid chromatography. These approaches provide definitive identification of the relevant hormone or compound according to its chemical or physical char- acteristics (e.g., unequivocal detection of performance- enhancing agents in sports). Other blood tests that are routinely used in endo- crine disorders include various autoantibodies. For example, antithyroid peroxidase (anti-TPO) antibod- ies are measured during the initial diagnostic workup and subsequent follow-up of patients with Hashimoto thyroiditis. Other endocrine disorders that use autoan- tibody testing include type 1 diabetes, Graves disease, autoimmune hypoparathyroidism, and autoimmune Addison disease. UrineTests Measurements of urinary hormone or hormone metab- olites often are done on a 24-hour urine sample and provide a better measure of hormone levels during that period than hormones measured in an isolated blood sample. The advantages of a urine test include the rela- tive ease of obtaining urine samples and the fact that blood sampling is not required. The disadvantage is

that reliably timed urine collections often are difficult to obtain. For example, a person may be unable to uri- nate at specific timed intervals, and urine samples may be accidentally discarded or inaccurately preserved. Because many urine tests involve the measurement of a hormone metabolite rather than the hormone itself, drugs or disease states that alter hormone metabolism may interfere with the test result. Some urinary hormone metabolite measurements include hormones from more than one source and are of little value in measuring hor- mone secretion from a specific source. For example, uri- nary 17-ketosteroids are a measure of both adrenal and gonadal androgens. Stimulation and SuppressionTests Stimulation tests are used when hypofunction of an endocrine organ is suspected. Atropic or stimulating hormone can be administered to test the capacity of an endocrine organ to increase hormone production. The capacity of the target gland to respond is measured by an increase in the appropriate hormone. For example, the function of the hypothalamic-pituitary-adrenal system can be evaluated through stimulation tests using ACTH and measuring the cortisol response. Failure to increase cortisol levels after a ACTH stimulation test suggests an inadequate capacity to produce cortisol by the adrenals (i.e., the adrenal is dysfunctional in some way). Suppression tests are used when hyperfunction of an endocrine organ is suspected. When an organ or tissue is functioning autonomously (i.e., is not responding to the normal negative feedback control mechanisms and continues to secrete excessive amounts of hormone), a suppression test may be useful to confirm the situation. For example, when a GH-secreting tumor is suspected, the GH response to a glucose load is measured as part of the diagnostic workup (see Chapter 32). Normally, a glucose load would suppress GH levels. However, in adults with GH-secreting tumors (a condition known as acromegaly ), GH levels are not suppressed (and para- doxically increase in 50% of cases). GeneticTests The diagnosis of genetic diseases using DNA analysis is rapidly becoming a routine part of endocrine prac- tice. Completion of the human genome sequence has revealed the presence of about 30,000 genes. The con- siderable interest in the field of genomics (i.e., examina- tion of the DNA) and transcriptomics (i.e., examination of the mRNA) has been complemented by advances in proteomics (i.e., examination of the proteome, which is all of the proteins expressed by a cell or tissue type). It is proposed that compared with the size of the genome, the proteome is far larger, with several hundred thou- sand to several million different protein forms possible. Analysis of the proteins produced by normal and abnor- mal endocrine cells, tissues, and organs will lead to a better understanding of the pathophysiologic processes of endocrine conditions. This may also lead to selective targeting for new drug development.