Function of Hormones

The physiologic functions of hormones can be divided into three general areas: (1) growth and differentiation, (2) maintenance of homeostasis, and (3) reproduction.

Growth

  Multiple hormones and nutritional factors mediate the complex phenomenon of growth. Short stature may be caused by GH deficiency, hypothyroidism, Cushing’s syndrome, precocious puberty, malnutrition, chronic illness, or genetic abnormalities that affect the epiphyseal growth plates (e.g., FGFR3 or SHOX mutations). Many factors (GH, IGF-I, TH) stimulate growth, whereas others (sex steroids) lead to epiphyseal closure. Understanding these hormonal interactions is important in the diagnosis and management of growth disorders. For example, delaying exposure to high levels of sex steroids may enhance the efficacy of GH treatment.

Maintenance of Hormone

   Though virtually all hormones affect homeostasis, the most important among these are the following: 
1. TH controls about 25% of basal metabolism in most tissues. 
2. Cortisol exerts a permissive action for many hormones in addition to its own direct effects. 
3. PTH regulates calcium and phosphorus levels. 
4.  Vasopressin regulates serum osmolality by controlling renal free-water clearance. 
5. Mineralocorticoids control vascular volume and serum electrolyte (Na+, K+) concentrations. 
6.  Insulin maintains euglycemia in the fed and fasted states.

The defense against hypoglycemia is an impressive example of integrated hormone action. In response to the fasted state and falling blood glucose, insulin secretion is suppressed, resulting in decreased glucose uptake and enhanced glycogenolysis, lipolysis, proteolysis, and gluconeogenesis to mobilize fuel sources. If hypoglycemia develops (usually from insulin administration or sulfonylureas), an orchestrated counterregulatory response occurs glucagon and epinephrine rapidly stimulate glycogenolysis and gluconeogenesis, whereas GH and cortisol act over several hours to raise glucose levels and antagonize insulin action.

Although free-water clearance is primarily controlled by vasopressin, cortisol and TH are also important for facilitating renal tubular responses to vasopressin. PTH and vitamin D function in an interdependent manner to control calcium metabolism. PTH stimulates renal synthesis of 1,25-dihydroxyvitamin D, which increases calcium absorption in the gastrointestinal tract and enhances PTH action in bone. Increased calcium, along with vitamin D, feeds back to suppress PTH, thereby maintaining calcium balance.

Depending on the severity of a given stress and whether it is acute or chronic, multiple endocrine and cytokine pathways are activated to mount an appropriate physiologic response. In severe acute stress such as trauma or shock, the sympathetic nervous system is activated and catecholamines are released, leading to increased cardiac output and a primed musculoskeletal system. Catecholamines also increase mean blood pressure and stimulate glucose production. Multiple stress-induced pathways converge on the hypothalamus, stimulating several hormones including vasopressin and corticotropin-releasing hormone (CRH).These hormones, in addition to cytokines (tumor necrosis factor α, IL-2, IL-6), increase ACTH and GH production. ACTH stimulates the adrenal gland, increasing cortisol, which in turn helps to sustain blood pressure and dampen the inflammatory response. Increased vasopressin acts to conserve free water.

Reproduction
The stages of reproduction include 
1) sex determination during fetal development; 
2) sexual maturation during puberty; 
3) conception, pregnancy, lactation, and child-rearing; 
4) cessation of reproductive capability at menopause. 
  Each of these stages involves an orchestrated interplay of multiple hormones, a phenomenon well illustrated by the dynamic hormonal changes that occur during each 28-day menstrual cycle. In the early follicular phase, pulsatile secretion of LH and FSH stimulates the progressive maturation of the ovarian follicle. This results in gradually increasing estrogen and progesterone levels, leading to enhanced pituitary sensitivity to GnRH, which, when combined with accelerated GnRH secretion, triggers the LH surge and rupture of the mature follicle. Inhibin, a protein produced by the granulosa cells, enhances follicular growth and feeds back to the pituitary to selectively suppress FSH, without affecting LH. Growth factors such 9 as EGF and IGF-I modulate follicular responsiveness to gonadotropins. Vascular endothelial growth factor and prostaglandins play a role in follicle vascularization and rupture.

     During pregnancy, the increased production of prolactin, in combination with placentally derived steroids (e.g., estrogen and progesterone), prepares the breast for lactation. Estrogens induce the production of progesterone receptors, allowing for increased responsiveness to progesterone. In addition to these and other hormones involved in lactation, the nervous system and oxytocin mediate the suckling response and milk release.