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Chapter 22: The Endocrine System

Alison Woodworth; Vipul Lakhani; Samir L. Aleryani; Michael Laposata

Learning Objectives

Learn the physiology and biochemistry of the relevant hormones and other important mediators.
Understand the laboratory tests used in the diagnosis of the more commonly encountered disorders.
Identify the clinical disorders associated with each of the endocrine glands and the role of specific laboratory tests in their diagnosis.

Introduction

This chapter on endocrine disorders is divided into separate discussions of each of the endocrine glands. Each section begins with an overview of the physiology and biochemistry of the relevant hormones. In addition, because of the large number of (often complex) laboratory tests in endocrinology, each section has a brief description of the laboratory tests most frequently used to diagnose the disorders in that disease group. Tests for which either serum or plasma is an acceptable specimen for analysis are noted as serum tests. Tests specifically requiring plasma are indicated by inclusion of the word “plasma” before the test name. As with all other chapters, each disorder is presented with a description of the disease and information useful in establishing a diagnosis. Figure 22–1 shows a general approach to the patient with endocrine disease.

Figure 22–1

An approach to the patient with an endocrinologic disorder.

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Thyroid

Physiology and Biochemistry

Production of thyroid hormones is regulated by the hypothalamic–pituitary–thyroid axis (Figure 22–2). Thyrotropin-releasing hormone (TRH) is produced in the hypothalamus and induces thyroid-stimulating hormone (TSH or thyrotropin) production in the anterior pituitary. TSH, in turn, stimulates thyroid hormone production and release by the thyroid gland. TSH production is inversely related to plasma thyroxine (T₄) and triiodothyronine (T₃) concentrations. The 2 primary hormones synthesized and secreted by the thyroid gland are T₄ and, in lesser quantities, T₃ (Figure 22–3). They are transported by plasma proteins—notably thyroid-binding globulin (TBG), transthyretin, and albumin—to various tissue sites where T₄ is deiodinated to the active form, T₃, and the inactive form known as reverse T₃ (rT₃). Thyroid hormones act through nuclear hormone receptors that are transcription factors for numerous genes. These genes regulate a number of critical physiologic functions in development and metabolism.

A “generational” classification has been applied for TSH assays based on the assay sensitivity. Third-generation assays can accurately measure TSH as low as 0.01 mU/L.