| Hormones of the hypothalamo-anterior pituitary regulatory system
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| There are five separate endocrine axes within this system (Fig. 37.4). Three of these are part of a complex three-level endocrine axis in which the hormones of the pituitary gland (thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) may be regarded solely as tropic hormones for other target organs (i.e. thyroid, adrenals, gonads). In these axes, sophisticated control is exercised via a cascade in which each endocrine organ in the axis amplifies both the amount of hormone secreted and the biological half-life of its hormone product compared with the previous organ. The fourth endocrine axis is a hybrid: growth hormone (GH) is both a tropic hormone and has actions in its own right. The fifth endocrine axis mediates the secretion of prolactin, unique in that it is not a tropic hormone. With one exception, there are distinct endocrine disorders associated with both deficiency and excess secretion by the anterior pituitary hormones (Table 37.2).
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