Assessment of Endocrine Function

Abstract

This chapter describes the various methods used for quantifying concentrations of circulating hormones and thus assessing endocrine function. The paradigm of feedback regulation (for example, of the hypothalamic-pituitary-target gland axis) is central to this assessment of endocrine status. Any disruption in such an axis can cause alterations in trophic and target hormone pairs. High concentration of a target gland hormone coupled with low concentration of the corresponding trophic hormone (e.g., pituitary hormone) suggests autonomous secretion by the target endocrine organ, as is typical in primary hyperthyroidism, e.g., high thyroxine (T4), suppressed thyroid stimulating hormone (TSH). Elevated concentrations of both members of a hormone pair usually indicate autonomous secretion of the trophic hormone, either from the normal site or from a tumor in an “ectopic” (extraglandular) location. For example, excess cortisol production driven by a high plasma adrenocorticotropic hormone (ACTH) level may be due to the secretion of pituitary ACTH or secretion of ACTH by lung tumors. Alternatively, the combined elevation of trophic and target endocrine gland hormones can result from resistance to the action of the target endocrine gland hormone e.g., elevated luteinizing hormone (LH) and testosterone in androgen resistance. Autonomous hypersecretion of the trophic hormone typically results in clinical evidence of target gland hormone excess, whereas resistance to the target gland hormone leads to manifestations of hormone deficiency. Hormones circulating in the plasma were first detected by in vivo bioassays, in which plasma or extracts of plasma were injected into animals and biological responses were measured. Unfortunately, most in vivo bioassays lack the precision, sensitivity, and specificity required to measure the low concentrations of many hormones in plasma, and the assays are cumbersome and impractical for routine use in clinical chemistry laboratories. Great progress in measuring plasma hormone concentrations came with the development of radioimmunoassays (RIAs) in the late 1950s. An unknown concentration of hormone in plasma is estimated by allowing competition with a labeled hormone or analog for specific binding sites on an antibody.