Endocrinology

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The pituitary gland (hypophysis) is an endocrine organ located at the base of the skull in a bony recess, called the sella turcica, consisting primarily of an anterior (adenohypophysis) and posterior (neurohypophysis) lobe. Collectively, under the influence of the hypothalamus, these lobes control the hormone secretions responsible for growth, reproduction, behaviour/ emotion, metabolism, and homeostasis, via a complex interplay of feedback loops. Dysfunction through failed synthesis of hormones, ‘breaks’ in the feedback pathways, or receptor malfunction can have diverse effects on plasma hormone levels and, hence, end organ function. The hypothalamus is located within the base of the third ventricle in the diencephalon and is responsible for maintaining homeostasis, as well as influencing emo­tion and behaviour. It is linked to the pituitary gland, which sits outside the dura, via the pituitary stalk and the hypothalamic– hypophyseal portal system. The anterior pituitary lobe makes up about 80% of the pituitary gland and is linked indirectly to the hypothalamus. It receives hormones released from neurosecretory cells in the paraventricular region of the hypothalamus, via a dense network of capillaries that make up the hypothalamic– hypophyseal portal system. These hormones subsequently bind to specific receptors on the pituitary cells to regulate a number of physio­logical processes including stress, growth, metabolism, reproduction, and lactation. There are six hormones re­leased by the anterior pituitary— adrenocorticotropic hormone (ACTH), growth hormone (GH), thyroid-stimulating hormone (TSH), follicle- stimulating hor­mone (FSH)/ luteinizing hormone (LH), and prolactin (PRH) (see Table 4.1). The posterior pituitary lobe is controlled via axons and nerve terminals that extend down from the hypothalamus through the pituitary stalk and into the lobe, which sub­sequently releases neurohormones (oxytocin and vaso­pressin) into the blood stream (see Table 4.2). Vasopressin (also called antidiuretic hormone, ADH), is essential in maintaining fluid homeostasis to ensure adequate blood volume and salt concentration. Its release is modulated by osmoreceptors (rising osmolality) in the hypothal­amus and baroreceptors (falling BP) in the cardiovascular system, and acts directly on the distal nephron to ensure water is conserved. Dysregulation of ADH may lead to conditions such as diabetes insipidus (DI) or SIADH (syn­drome of inappropriate ADH).