Clinical Endocrinology and Diabetes at a Glance

Aled Rees, Miles Levy, Andrew Lansdown

• Publisher: Wiley-Blackwell
• Year: 2017
• ISBN:
  9781119128731
• Edition: 1
• Topic: Medicine
• Subtopic: Endocrinology & Metabolism

Clinical Endocrinology and Diabetes at a Glance

The market?leading at a Glance series is popular among healthcare students, and newly qualified practitioners for its concise and simple approach and excellent illustrations.

Each bite?sized chapter is covered in a double?page spread with clear, easy?to?follow diagrams, supported by succinct explanatory text.

Covering a wide range of topics, books in the at a Glance series are ideal as introductory texts for teaching, learning and revision, and are useful throughout university and beyond.

Everything you need to know about Clinical Endocrinology and Diabetes… at a Glance!

Recognizing a need to promote and enhance teaching of clinical endocrinology and diabetes in UK medical schools, the Society for Endocrinology, in collaboration with Diabetes UK and the Association of British Clinical Diabetologists, created a national curriculum that sets out the minimum recommended standards for undergraduate medical education.

Clinical Endocrinology and Diabetes at a Glance corresponds to the curriculum and covers all important areas of diabetes and endocrinology in a clinically relevant and concise manner, with complementary figures to ensure principles are explained clearly. It highlights key practical skills for undergraduates in line with the GMC recommendations that medical graduates should be well prepared for in their Foundation posts. There is also emphasis on the importance of multidisciplinary teams in the management of endocrine diseases and diabetes. These include sections on structured education programmes, psychosocial aspects, and patient self-management and monitoring.

Clinical Endocrinology and Diabetes at a Glance will serve as a valuable resource for medical students and junior doctors treating patients with these conditions

Part 1
Introduction

Chapter

1. 1 Introduction to endocrinology

1
Introduction to endocrinology

The endocrine system consists of glands, which secrete hormones that circulate and act at distant sites in the body. The key endocrine glands are the pituitary, thyroid, parathyroids, adrenals, pancreas and gonads. Endocrine disease can lead to hypo- or hypersecretion of hormones. Endocrine diseases include tumours, which are commonly benign, autoimmune diseases, enzyme defects and hormone receptor abnormalities.

Synthesis, release and transport

The chemical structure of hormones includes steroids, polypeptides, glycoproteins and amines (Figure 1.1). Hormones are secreted by the hypothalamus at low concentration, acting locally on the anterior pituitary, which in turn secretes trophic hormones to the relevant target gland. Hormones are secreted directly into the circulation either in their final form or as a larger precursor molecule, such as proopiomelanocortin (POMC), which is cleaved to adrenocorticotrophic hormone (ACTH), melanocyte stimulating hormone (MSH) and other smaller peptides. Many hormones are transported in the circulation by binding proteins, but only the free hormone acts on the receptor. Examples of binding proteins are sex hormone binding globulin (SHBG), which binds testosterone, and cortisol binding globulin (CBG), which binds cortisol.

Mechanisms of hormone action

Cell-surface receptors

Peptide hormones act on cell-surface receptors and exert their effect by activating cyclic adenosine monophosphate (cAMP). Most peptide hormones act via G-protein coupled receptors, most commonly a 7-trans-membrane (7TM) receptor (Figure 1.1). Examples of peptide hormones are growth hormone (GH), thyroid stimulating hormone (TSH), prolactin and ACTH.

Intranuclear receptors

Lipid-soluble hormones such as steroids and thyroid hormones pass through the cell membrane and act on intranuclear receptors, causing altered gene transcription (Figure 1.1).

Control and feedback

Hormones are usually controlled by a negative feedback mechanism (Figure 1.1). Using the thyroid axis as an example, the hypothalamus secretes its thyrotrophin releasing hormone (TRH), which travels down the portal tract to act on the anterior pituitary. The pituitary releases its trophic hormone (TSH) into the circulation, which acts on the target gland, stimulating the production of the relevant hormone (thyroxine). If the target gland hormone is too low, there is loss of negative feedback and a compensatory increase in the pituitary hormone (low T4, high TSH). If the target gland hormone is too high, there is increased negative feedback and suppression of the pituitary hormone (high T4, low TSH). All pituitary hormones are under predominantly stimulatory control by the hypothalamus apart from prolactin, which is under tonic inhibition by dopamine.

Patterns of hormone secretion

Some hormones are produced in a stable pattern with little circadian rhythmicity, for example thyroxine and prolactin. Other hormones have a significant diurnal variation. For example, cortisol is highest in the morning and lowest at midnight. Minor circadian rhythms can be seen with certain hormones such as testosterone, which is slightly higher in the morning than the afternoon. It is important to measure hormones at the appropriate time of day when assessing for deficiency or excess. Female hormones have a monthly cyclical variation and must be interpreted according to the time of the menstrual cycle.

Measurement of hormones

Hormones are usually measured by immunoassay, which uses specific labelled antibodies that give a signal according to the concentration of hormone. Interfering antibodies can affect blood results, so some results are not reflective of the true concentration of hormone. Assay interference should be suspected in any blood result that does match the clinical picture. Mass spectrometry is a newer technique that provides a more specific measure, and is increasingly being adopted in endocrine laboratories.

Dynamic endocrine tests

When basal investigations are difficult to interpret because of diurnal variation or equivocal results, 24-hour urine collection or dynamic blood tests can be helpful. If hormone deficiency is suspected, a stimulation test is used. This involves administration of a hormone that stimulates the target gland to increase its hormone secretion. Examples are the Synacthen test (to stimulate cortisol in suspected primary adrenal failure) and the insulin tolerance test (to stimulate GH and ACTH in suspected hypopituitarism). If hormone excess is suspected, a suppression test is used. Examples are the dexamethasone suppression test (to suppress cortisol in suspected Cushing’s syndrome) and the oral glucose tolerance test (to suppress GH in suspected acromegaly).

Part 2
Disorders of the hypothalamic–pituitary axis

Chapters

1. 2 The hypothalamic–pituitary axis and its assessment
2. 3 Acromegaly
3. 4 Cushing’s syndrome
4. 5 Hypopituitarism and non-functioning pituitary adenomas
5. 6 Prolactinoma and hyperprolactinaemia

2
The hypothalamic–pituitary axis and its assessment

The pituitary gland is the ‘conductor of the endocrine orchestra’, controlling all peripheral glands via trophic hormones. It is approximately the size of a pea and sits in the pituitary fossa at the base of the brain (Figure 2.1). The anterior pituitary is derived embryologically from Rathke’s pouch, derived from primitive gut tissue. The posterior pituitary is derived from a down-growth of primitive brain tissue. The optic chiasm lies superior to the pituitary gland. Lateral is the cavernous sinus, which contains cranial nerves III, IV and Va and the internal carotid artery (Figure 2.1).

Physiology

Hypothalamic releasing and inhibiting factors are t...