Review of Medical Physiology - William F. Ganong 2002

Endocrine System, Metabolism, and Reproduction
Pituitary Gland
Hypopituitarism

Changes in Other Endocrine glands

Surgical removal of the Pituitary Gland or its destruction by disease in humans or animals frequently results in changes similar to those seen in other glandular hormone deficiencies. Hypopituitarism leads to Atrophy of the adrenal cortex, resulting in a significant decrease in the secretion of adrenal glucocorticoids and sex Steroids, although a low level of basal secretion persists. Stress does not stimulate aldosterone output, but basal aldosterone secretion and its increase in response to salt depletion remain within normal limits for some time. Because mineralocorticoid deficiency does not occur, salt wasting and hypovolemic Shock do not develop. However, the inability to upregulate glucocorticoid secretion renders patients with hypopituitarism highly susceptible to stress. The Development of a salt-wasting state due to prolonged hypopituitarism is discussed in Chapter 20. Growth in these cases is stunted (see above). Thyroid function is markedly diminished, predisposing the individual to cold intolerance. Atrophy of the Gonads also occurs, leading to the cessation of reproductive cycles and the loss of certain secondary sexual characteristics.

Insulin Sensitivity

Hypophysectomized animals show a tendency toward hypoglycemia, especially when obese. In some species, though not in humans, fatal hypoglycemic reactions are quite common. Hypophysectomy ameliorates the symptoms of Diabetes Mellitus (see Chapter 19) and significantly potentiates the hypoglycemic effect of insulin. This is partly due to adrenocortical hormone deficiency; however, hypophysectomized animals are more sensitive to insulin than adrenalectomized ones because they also lack the anti-insulin effect of Growth Hormone.

Class="center">

Figure 22-16. Growth curve for a normal boy who was ill from age 5 to 7. During the catch-up growth period, he regained a height appropriate for the normal growth curve (Modified from Boersma B, Wit JM: Catch-up growth. Endocr Rev 1997; 18:646).

Water METABOLISM

Although a selective Impairment of the supraoptic-posterior pituitary system causes diabetes insipidus (see Chapter 14), the removal of both the anterior and posterior lobes of the gland typically leads only to transient polyuria. It was previously hypothesized that the anterior pituitary secretes a diuretic hormone, but the amelioration of diabetes insipidus symptoms is actually explained by a reduction in the solute load requiring excretion. Osmotically active particles retain water within the renal tubules (see Chapter 38). ACTH deficiency in hypophysectomized animals causes a decreased rate of Protein Catabolism. Due to TSH deficiency, the metabolic rate is similarly depressed. Consequently, fewer osmotically active catabolic products are filtered, and urine volume decreases even in the absence of vasopressin. Growth hormone deficiency leads to a reduced Glomerular Filtration rate in hypophysectomized animals; conversely, growth hormone increases glomerular filtration rate and renal plasma flow in humans. Glucocorticoid deficiency results in impaired water clearance identical to that seen in adrenalectomized animals. The apparent diuretic activity of the anterior pituitary can thus be accounted for by the actions of ACTH, TSH, and growth hormone.

Other Abnormalities

Deficiencies of ACTH and other Pituitary Hormones that stimulate melanocyte-stimulating hormone (MSH) are responsible for the pallor of the Skin in patients with hypopituitarism. Some protein loss may occur in adults, although emaciation is not a hallmark of human hypopituitarism, and most patients with pituitary insufficiency are actually overweight (Figure 22-17). Although cachexia was once considered part of the clinical picture, it is now recognized that the emaciated patients described in older literature most likely suffered from anorexia nervosa rather than hypopituitarism.

Causes of Pituitary Insufficiency in Humans

Anterior pituitary tumors are classified based on their staining affinities into chromophobe, acidophil, or basophil adenomas. Many apparently non-functioning tumors are chromophobes that cause hypopituitarism by destroying normal pituitary tissue. Approximately 70% of patients with chromophobe tumors exhibit elevated Blood levels of prolactin. This indicates that prolactin-secreting tumors also exert pressure on the Hypothalamus or the pituitary stalk, disrupting The transport of dopamine from the hypothalamus to the anterior pituitary. Suprasellar cysts, enlarging remnants of Rathke's pouch that compress the pituitary gland, and various other disorders can likewise lead to hypopituitarism. Women who experience shock secondary to postpartum Hemorrhage may develop pituitary infarction followed by subsequent postpartum necrosis (Sheehan's syndrome). The blood supply of the anterior pituitary is particularly vulnerable because the gland descends onto the pituitary stalk through the rigid diaphragma sellae, and the pituitary normally enlarges during Pregnancy. Pituitary infarction in men is rare; however, it has been observed among soldiers who contracted hemorrhagic fever in Korea. This disease is characterized by a diffuse vasculitis that apparently causes the pituitary to swell due to edema, and the patients who developed pituitary infarction were those who went into shock during the course of the illness.

Figure 22-17. Typical appearance in adult hypopituitarism. Note the normal body habitus and skin pallor (Reproduced with permission from Daughaday WH¹ The adenohypophysis. In: Textbook of Endocrinology, 5th ed. Williams RH [editor]. Saunders, 1974).