The hypothalamus controls many endocrine systems either directly or through the pituitary gland. Higher central nervoussystem (CNS) centers control the hypothalamus. Hypothalamicreleasing or inhibiting factors travel through capillaries of the pituitary portal system to control the anterior pituitary gland,regulating the hormones specific for the factor (Fig. 170-2).The pituitary hormones enter the peripheral circulation andexert their effects on target glands, which produce other hormones that feed back to suppress their controlling hypothalamicand pituitary hormones. Insulin-like growth factor-1 (IGF-1),growth hormone (GH), cortisol, sex steroids, and thyroxine (T4)all feed back on the hypothalamic-pituitary system. Prolactin isthe only pituitary hormone that is suppressed by a hypothalamicfactor, dopamine. The hypothalamus is also the location of vasopressin-secreting axons that either terminate in the posteriorpituitary gland and exert their effect via vasopressin secretion
Figure 170-1 Schematic representation of mechanisms of action of hormones and growth factors. Although traditional hormones are formed in endocrine glands and transported to distant sites of action through the bloodstream (endocrine mechanism), peptide growth factors may be produced locally by the target cells themselves (autocrine modality of action) or by neighboring cells (paracrine action). (From Wilson JD, Foster DW, editors: Williams Textbook of Endocrinology, ed 8, Philadelphia, 1992, WB Saunders, p 1007.)
HYPOTHALAMUSPITUITARYSRIF (somatostatin) GHRH CRH GH ACTH Prolactin FSH, LH TSH Dopamine GnRH TRH Growth Cortisol Lactation T4, T3 Sex steroids Insulin-like growth factors (and binding proteins)
Figure 170-2 Hormonal influences of the hypothalamus and pituitary gland. Solid line represents stimulatory influence; dotted line represents inhibitory influence. ACTH, Adrenocorticotropic hormone; CRH, corticotropin-releasing hormone or CRF; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, GH-releasing hormone or GRF; GnRH, gonadotropin-releasing hormone or luteinizing hormone releasing factor, LRF or LHRH; LH, luteinizing hormone; SRIF, somatotropin release-inhibiting factor, somatostatin or SS; Inhibin TRH, thyrotropin-releasing hormone or TRF; TSH, thyroid-stimulating hormone; T3, triiodothyronine;
T4, thyroxine.
Table 170-1 Diagnostic Evaluation for Hypopituitarism MANIFESTATION CAUSE TESTS* Growth failure, hypothyroidism, or both GH deficiency, TRH/TSH deficiency, or both Provocative GH tests, free T4, bone age, IGF-1, IGFBP3
Hypoglycemia GH deficiency, ACTH insufficiency, or both Provocative GH tests, test of ACTH secretion, IGF1, IGFBP3
Polyuria, polydipsia ADH deficiency Urine analysis (specific gravity), serum electrolytes, urine and serum osmolality, water deprivation test