Alternative names for some transcription factors are given in parentheses
Hormones act by binding to receptors and triggering intracellular responses
Tissue distribution of the receptor determines where a hormone exerts its effect
The two major subdivisions of hormone receptor are classified by their cellular site of action: cell surface or nuclear
Peptide hormones and catecholamines act via cell‐surface receptors and generate fast responses in seconds or minutes
Steroid and thyroid hormones act via nuclear receptors to alter the expression of target genes, with subsequent translation into protein; the response is slow, most commonly over hours
Mutations in genes encoding any part of the cascade from hormone to hormone receptor and downstream signalling cascade can cause under or over‐activity, or, potentially, tumour formation
CHAPTER 4 Investigations in endocrinology and diabetes
Key topics
Pre‐analytical requirements for accurate endocrine testing
Cell and molecular biology as diagnostic tools
Learning objectives
To understand how circulating hormones are measured
To understand how other laboratory investigations are applied to clinical endocrinology and diabetes
To understand the molecular biology that underpins genetic diagnoses
To understand the options available for imaging the endocrine system
This chapter details how clinical endocrinology and diabetes is investigated
All specialties have been advanced by methods to aid diagnosis, and monitor and assess treatment. Investigation in endocrinology and diabetes remains centred on laboratory assays that determine the concentration of hormones and metabolites, usually in blood. In addition to clinical biochemistry (also called chemical pathology), molecular genetics and cytogenetics are routine investigations to provide personalized genetic diagnoses that predict the course of some endocrine disorders (e.g. multiple endocrine neoplasia; Chapter 10).
Outside of the laboratory, clinical investigation draws heavily upon expertise in radiology and nuclear medicine. Some investigations are highly specific (e.g. visual fields for pituitary tumours or retinal screening for diabetes) and these are covered in later topic‐specific chapters.
Pre‐analytical requirements for accurate endocrine testing
Before any sample is measured, it is important that the endocrine testing has been performed using standard protocols to reduce the risk of misleading laboratory results. This involves ensuring that the correct test is performed with the patient properly prepared. It is important that clear instructions are given to the patient about steps that need to be taken prior to the test, e.g. fasting (Box 4.1). Care must be taken with sample collection, transport and handling in the laboratory.
Laboratory assay platforms
Hormones (and other metabolites) are most commonly measured by immunoassay, although increasingly mass spectrometry is used. Immunoassays, introduced in the 1960s, are sufficiently sensitive, precise and hormone specific for routine application in clinical biochemistry. Bioassays, which measure physiological responses induced by a stimulus, are near obsolete in clinical practice.
Immunoassays
Immunoassay is a broad term for one of two different techniques: true immunoassay and immunometric assay. Both forms are based on the premise that the hormone to be measured is antigenic and can be bound by specific antibodies to form an antibody–antigen complex. Both forms of immunoassay also employ a label to generate a quantitative signal. Historically a radioactive isotope [e.g. iodine‐125 (I125)] was used, but now non‐radioactive methods, commonly using a fluorescent tracer, are employed. Both assays also rely on comparison of the patient sample with known concentrations of a reference compound.
To set up a calibration or standard curve for the immunoassay, a constant amount of antibody is added to a series of tubes with increasing, known amounts of a reference preparation; the example in Figure 4.1 is growth hormone (GH). This reaction is reversible with the antigen and antibody continuously associating and dissociating; however, after incubation, equilibrium is reached when tubes with more GH generate more bound complex. Measurement of the amount of bound complex (e.g. in terms of fluorescence or radioactivity) can thus be related to the quantity of GH that was originally added. This allows a calibration curve to be plotted, against which the same process can now deduce the GH concentration in a patient sample.
Box 4.1 Pre‐analytical requirements for accurate endocrine testing
Patient preparation:
Prior fasting may be requiredGlucoseLipidsCalcium
Dietary
