Clinical Physiology of Endocrine Disorders Understanding the Signs and Symptoms Raji Subramanian, Lam Sau Kuen
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Introduction to the Endocrine System1

 
THE ENDOCRINE SYSTEM
The main endocrine glands in the body are the hypothalamus, pituitary, thyroid, parathyroids, pancreatic islets, adrenals, gonads and placenta. They secrete substances called hormones directly into the circulation. Some of these hormones are also released from nerve endings where they act as neurotransmitters or neuromodulators.
In addition, recently, almost all organs and tissues in the body, like the heart, kidney, liver, gastrointestinal (GI) and adipose tissues have also been shown to possess endocrine activity.
 
Functions
The endocrine system, in conjunction with the nervous system, serves as a homeostatic control mechanism to regulate bodily functions such as:
  • Maintenance of the internal milieu
  • Control of storage and utilisation of energy substrates
  • Regulation of growth, development and reproduction
  • Adaptation to stress.
 
Hormones
Hormones are classified into three classes:
  • Proteins, polypeptides and glycoproteins (hypothalamic and pituitary hormones).
  • Steroids (reproductive and adrenocortical hormones).2
  • Amino acid derivatives (thyroid and adrenomedullary hormones).
By definition, hormones are substances released directly into the circulation to exert their effects on distant target tissues or organs (endocrine effects). However, they can also act locally on neighbouring tissues (paracrine effects) or directly on the cells that produce them (autocrine effects).
 
Transport of Hormones in Circulation
In circulation, hormones are usually present in minute concentrations in the range of nanograms or picograms. In general, the peptide hormones circulate unbound in the plasma, while only a small fraction of steroid and thyroid hormones are transported freely, and the rest are bound to plasma proteins. The effect of the hormones on target tissues is mediated by the free form, while the bound form acts as a reservoir. The concentration of the unbound hormones in circulation is dependent on the amount that is secreted, the amount that is bound to proteins and the amount that is degraded.
 
Mechanism of Action at Target Tissues
Peptide and adrenomedullary hormones, which do not readily cross the plasma membrane, generally bind to cell-surface receptors, thus activating the second messenger systems to bring about their actions. In contrast, steroid and thyroid hormones easily penetrate the cell membrane and bind to cytoplasmic or nuclear receptors to produce their effects.
 
Feedback Control
The immediate stimulus for hormonal secretion is neural, hormonal or humoral (the plasma level of a metabolite or an electrolyte). The rate of hormone synthesis and secretion is precisely controlled by negative or positive feedback mechanisms. In the body, most of the control is exerted through negative feedback mechanisms, while positive feedback is mainly seen in the regulation of ovulation, parturition and lactation.3
 
Clinical Conditions
Endocrine disorders occur because of either an under-or over-secretion or because of receptor resistance of the respective hormones.
The following diagrams illustrate the hormonal, neural and humoral control and feedback mechanisms involved in hormone secretion.
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Fig. 1.1: Hormonal feedback control mechanism: Growth hormone, thyroid hormones, adrenocortical hormones, gonadal hormones
4
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Fig. 1.2: Neural feedback control mechanism: Prolactin, oxytocin, antidiuretic hormone, adrenomedullary hormones
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Fig. 1.3: Humoral feedback control mechanism: Insulin, glucagon, parathyroid hormone, calcitriol, calcitonin