USMLE – Endocrine Diseases
The body has two major control systems which allow specialized tissues to function in an integrated way: the nervous system and the endocrine system. Endocrinology concerns the synthesis, secretion and action of hormones. These are chemical messengers which coordinate the activities of different cells.
There are a large number of diseases of endocrine glands, but World Health Organization statistics indicate that they are a relatively rare cause of death (1-5%). Some endocrine diseases are common, particularly those of the thyroid gland, reproductive system and the beta cells of the pancreas. For example, thyroid dysfunction occurs in > 10% of the population in areas with iodine deficiency, e.g. the Himalayas, and 4% of women aged 20-30 years in the US. However, even rare endocrine syndromes cause important morbidity and present a particular diagnostic challenge to primary care clinicians who may see very few such patients during their working lives.
MAJOR ENDOCRINE FUNCTIONS AND ANATOMY
The classical model of endocrine function involves regulation of hormones which are synthesized in endocrine glands, released into the circulation and act at sites distant from those of secretion. However, additional levels of complex regulation have now been recognized. Thus, most major organs secrete hormones or contribute to the peripheral metabolism and activation of prohormones; many hormones act on adjacent cells (paracrine system, e.g. neurotransmitters), or even back on the cell of origin (autocrine system); and the sensitivity of target tissues is regulated in a tissue-specific fashion.
A wide variety of molecules act as hormones, including peptides, glycoproteins, steroids, amines and even simple compounds (e.g. nitric oxide). Some act on cell surface receptors and others require to be transported to the nucleus to activate gene transcription. The speed of these processes is very different, contributing to the contrasting clinical presentation of endocrine disease. For example, diseases of the thyroid and adrenal cortex are often chronic, while excess secretion of catecholamines (pheochromocytoma) or 5-hydroxytryptamine-5-HT, serotonin-(carcinoid syndrome) produces dramatic acute symptoms.