Endocrine System

Chapter 10

Endocrine System

Physiology of the endocrine system

The endocrine system is a biochemical communication network through which several small glands control a broad range of vital body activities. The endocrine glands secrete chemical messengers called hormones, which circulate in the blood and may affect a single target organ or the entire body. Hormones may be proteins (growth hormone), steroids (cortisone), peptides (antidiuretic hormone [ADH]), amino acids (thyroxine), or amines (epinephrine). They range from small to large molecules and have chemical structures of various complexities.

The major endocrine glands are the pituitary, adrenal, thyroid, and parathyroid glands. Inadequate (hypoactive) or excess (hyperactive) production of hormones from these endocrine glands can give rise to a wide variety of clinical symptoms and radiographic abnormalities.

Because hormones are powerful chemicals, it is essential that their circulating levels be carefully controlled. One type of control is called the negative feedback mechanism. In this system, an adequate level of a hormone in the blood automatically stops the release of additional hormone (somewhat like a thermostat). As the blood level of the hormone decreases, the gland is stimulated to secrete more of it. Another control mechanism is the production of two different hormones whose actions are opposite to each other. For example, insulin is secreted by the pancreas when the blood glucose level rises. When the blood glucose level falls below normal, a second hormone, glucagon, is secreted by the pancreas to raise the blood glucose level. Thus these two hormones are balanced so that a proper blood glucose level is continually maintained.

Adrenal glands

Physiology of the Adrenal Glands

Each of the adrenal glands, which are situated at the top of each kidney, consists of an outer cortex and an inner medulla (Figure 10-1). The adrenal cortex secretes several different types of steroid hormones, which can be divided into three general groups. The mineralocorticoids (primarily aldosterone) regulate salt and water balance by controlling sodium retention and potassium excretion by the kidneys. The production of aldosterone is regulated primarily by the secretion of renin from specialized cells (the juxtaglomerular apparatus) in the kidney. Reduced blood volume (as in hemorrhage) causes low blood pressure, which is detected by the juxtaglomerular apparatus and eventually results in increased aldosterone secretion from the adrenal cortex.

Glucocorticoids (especially cortisone) regulate carbohydrate metabolism and are under the regulation of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. Cortisone also depresses the inflammatory response to almost all forms of injury, thus leading to its use in the treatment of trauma, rheumatoid arthritis, bursitis, and asthma, and as an immunosuppressive agent to help limit rejection after organ transplantation.

Androgens are sex hormones that tend to masculinize the body, to retain amino acids, and to enhance protein synthesis. It is these hormones that are used both illegally and unwisely by athletes in an attempt to increase body strength.

The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine. These fight-or-flight hormones are secreted in stress situations when additional energy and strength are needed. Epinephrine stimulates heart activity, raises blood pressure, and increases the level of blood glucose. By constricting some blood vessels and dilating others, epinephrine shunts blood to active muscles where oxygen and nutrients are urgently needed.

Diseases of the Adrenal Cortex

Cushing’s Syndrome

The excess production of glucocorticoid hormones in Cushing’s syndrome may be attributable to generalized bilateral hyperplasia of the adrenal cortex, or it may be a result of a functioning adrenal or even nonadrenal tumor. It can also be the result of the exogenous administration of cortisone. Excess secretion of glucocorticoid hormones mobilizes lipids and increases their level in the blood. This increase produces a characteristic obesity that is confined to the trunk of the body and is associated with a round, moon-shaped face and a pathognomonic fat pad that forms behind the shoulders (buffalo hump). Retention of salt and water results in hypertension.

Radiographic Appearance: Generalized enlargement of the adrenal glands is best demonstrated by computed tomography (CT), which shows thickening of the wings of the adrenal gland, which appear to have a stellate or Y-shaped configuration in cross section. Ultrasound can also show diffuse adrenal gland enlargement.

Benign and malignant tumors of the adrenal cortex are less common causes of Cushing’s syndrome than is nontumorous adrenal hyperfunction. As a general rule, the larger the adrenocortical tumor and the more abrupt the onset of clinical symptoms and signs, the more likely the tumor is to be malignant (Figure 10-2). However, the differentiation between adenoma and carcinoma may be impossible at the time of histologic examination, and the nature of the tumor may have to be defined by the clinical course alone. Both CT and ultrasound can demonstrate an adrenal tumor (Figure 10-3), but CT is often more valuable because the abundance of retroperitoneal fat may prevent an optimal ultrasound examination. Adrenal venography has been widely used to demonstrate adrenal masses, and it also permits the aspiration of blood samples for assessment of the level of adrenal hormones.

Cushing’s syndrome produces radiographic changes in multiple systems. Diffuse osteoporosis causes generalized skeletal demineralization, which may lead to the collapse of vertebral bodies, spontaneous fractures, and aseptic necrosis of the head of the femur or humerus. Widening of the mediastinum as a result of excessive fat deposition sometimes develops in Cushing’s syndrome and can be confirmed by CT. Hypercalciuria caused by elevated steroid values can lead to renal calculi and nephrocalcinosis.

Imaging of the sella turcica by conventional tomography or CT is important in the routine assessment of a patient with Cushing’s syndrome. Most patients with nontumorous adrenal hyperfunction are found at surgery to have an intrasellar lesion. It is important to emphasize, however, that small pituitary microadenomas may be present in asymptomatic patients. The modality of choice to detect a functioning microadenoma causing adrenal hyperplasia is contrast-enhanced magnetic resonance imaging (MRI). A pituitary adenoma develops in up to one third of patients after adrenal surgery and produces progressive sellar enlargement. For this reason, yearly follow-up sellar tomograms may be indicated after adrenalectomy.

Nonpituitary tumors producing ACTH may cause adrenal hyperfunction and Cushing’s syndrome. The most common sites of origin are the lung, thymus, and pancreas; about half of these tumors can be demonstrated on chest radiographs. Octreotide scintigraphy (a nuclear medicine examination) detects ectopic ACTH tumors on the basis of increased uptake of tumor cell surface receptors for somatostatin.


An overproduction of mineralocorticoid hormones produced by the most superficial layer of the adrenal cortex causes retention of sodium and water and abnormal loss of potassium in the urine. This condition results in hypertension, muscular weakness or paralysis, and excessive thirst (polydipsia). Aldosteronism may be attributable to an adrenocortical adenoma (Conn’s syndrome) or to bilateral hyperplasia of the superficial cortical layer. Aldosteronism may also be the result of renin-secreting tumors, renal artery stenosis, malignant hypertension, and bilateral chronic renal disease. The biochemical assay is the basis for diagnosing aldosteronism, and CT or MRI is used for adrenal identification.

Radiographic Appearance: The role of diagnostic imaging is to demonstrate the location of adenomas that may otherwise be difficult to detect during exploratory surgery.

Noncontrast CT, the most widely used imaging modality, demonstrates the small adrenocortical adenoma as a contour abnormality of the gland (Figure 10-4). With use of CT or MRI, the adrenal gland can be measured quite accurately; the normal adrenal measures 3 to 6 mm thick, 4 to 6 mm long, and 2 to 3 cm wide. However, the scan or image may not demonstrate any abnormal findings. With newer CT scanners, specificity has increased to 75%, and tumors larger than 1 cm can be consistently identified. Adenomas may be isointense or hypointense relative to the liver on T1-weighted MR images. On T2-weighted images, they have slight hyperintensity. Chemical-shift imaging can aid in identifying and characterizing an adrenal mass. In adenomas smaller than 1 cm, nuclear medicine studies using I-131 attached with a cholesterol binder can differentiate a normal gland from hyperplasia on the basis of the time course of radionuclide uptake. Early uptake (less than 5 days) in both adrenal glands indicates hyperplasia, whereas unilateral early uptake implies an adrenal adenoma. Adrenal venography with biochemical assay of a sample of adrenal blood is another important technique for localizing aldosteronomas and determining whether the hyperaldosteronism is primary or secondary.

Adrenogenital Syndrome

The adrenogenital syndrome (adrenal virilism) is caused by the excessive secretion of androgenically active substances by the adrenal gland. In the congenital form, a specific enzyme deficiency that prevents the formation of androgenic hormones causes continuous ACTH stimulation and bilateral hyperplasia. The elevated levels of androgens result in accelerated skeletal maturation along with premature epiphyseal fusion, which may lead to dwarfism.

In women the syndrome causes masculinization, with the development of hair on the face (hirsutism). The breasts diminish, the clitoris enlarges, and ovulation and menstruation cease.


The clinical manifestations of adrenal insufficiency vary from those of a chronic insidious disorder (easy fatigability, anorexia, weakness, weight loss, and increased melanin pigmentation) to those of an acute collapse with hypotension, rapid pulse, vomiting, and diarrhea.

The most common cause of adrenal insufficiency is the excessive administration of steroids. Primary adrenocortical insufficiency (Addison’s disease) results from progressive cortical destruction, which must involve more than 90% of the glands before clinical signs of adrenal insufficiency appear. In the past, Addison’s disease was usually attributed to tuberculosis; at present most cases reflect idiopathic atrophy, probably on an autoimmune basis. In areas where the disease is endemic, histoplasmosis is an occasional cause of adrenal insufficiency.

Adrenal Carcinoma

About half of adrenal carcinomas are functioning tumors that cause Cushing’s syndrome, virilization, feminization, or aldosteronism. The tumors grow rapidly and are usually large necrotic masses at the time of clinical presentation.

Radiographic Appearance: Ultrasound demonstrates the tumor as a complex mass that may be difficult to separate from an upper pole renal tumor (Figure 10-7). CT demonstrates an adrenal carcinoma as a large unilateral mass with an irregular edge that often contains low-density areas resulting from central necrosis or prior hemorrhage (Figure 10-8) and high-density calcifications. On contrast-enhanced CT, the tumor enhancement is irregular and greatest on the periphery. For adrenal examinations, spiral CT with 3- to 5-cm section reconstructions offers the best resolution and may identify tumors 1 cm or smaller.

Because lymphatic and hepatic metastases are common at the time of clinical presentation, CT scans at multiple abdominal levels are necessary to define the extent of the primary tumor and to detect metastases before surgical resection is attempted. Extension of the tumor into the renal vein and inferior vena cava can also be detected by CT, especially after the injection of intravenous contrast material, or by MRI (Figure 10-9). On MR images, the higher signal intensity of the tumor in comparison with the liver on T2-weighted images (lower signal intensity on T1-weighted images) may distinguish adrenal carcinoma from nonfunctioning adenomas and pheochromocytomas.

Metastases to the Adrenal Gland

The adrenal gland is one of the most common sites of metastatic disease. The primary tumors that most frequently metastasize to the adrenal gland are carcinomas of the lung, breast, kidney, ovary, and gastrointestinal tract, and melanomas.

Radiographic Appearance: Metastatic enlargement of an adrenal gland can cause downward displacement of the kidney with flattening of the upper pole. Ultrasound and CT demonstrate adrenal metastases as solid, soft tissue masses that vary considerably in size and are frequently bilateral (Figure 10-10). However, the ultrasound (hypoechoic lesions) and CT patterns are indistinguishable from those of primary malignancies of the gland. Therefore, when a known primary tumor exists elsewhere, it is usually assumed that an adrenal mass is metastatic.

On MRI, metastases typically have higher signal intensity on T2-weighted images than do benign adenomas, and they also demonstrate increased contrast enhancement on T1-weighted, fat-suppressed images. In-phase and out-of-phase pulse sequences (also known as chemical-shift imaging) are highly accurate for distinguishing between adrenal adenomas and metastases. Lipid-laden adenomas show low signal intensity on out-of-phase images and intermediate to high signal intensity on in-phase images.

If necessary, a needle biopsy using ultrasound or CT guidance may be of value to determine whether the adrenal lesion is primary or metastatic.

Diseases of the Adrenal Medulla


A pheochromocytoma is a tumor that most commonly arises in the adrenal medulla and produces an excess of vasopressor substances (epinephrine and norepinephrine), which can cause an uncommon but curable form of hypertension. About 10% of pheochromocytomas are extraadrenal in origin. About 10% of patients with pheochromocytoma have bilateral tumors, and a similar percentage of pheochromocytomas are malignant.

Because in almost all patients the diagnosis of a pheochromocytoma can be made with biochemical tests, radiographic imaging serves as a confirmatory study and as a means of localizing the tumor. Excretory urography, even with nephrotomography, may be of limited value because the kidney is often not displaced even when the adrenal pheochromocytoma is large.

Radiographic Appearance: CT and ultrasound (Figure 10-11) are very useful in the localization of pheochromocytomas. The cross-sectional images not only detail the extent of the adrenal lesion but also define the status of adjacent structures and can demonstrate bilateral or

multiple pheochromocytomas, extraadrenal tumors, and metastases. Pheochromocytomas generally appear as round, oval, or pear-shaped masses, often greater than 3 cm, that are slightly less echogenic than liver and kidney parenchyma on ultrasound and have an attenuation value less than these organs on CT (Figure 10-12). Necrosis, hemorrhage, and fluid levels are common findings in larger lesions. Most extraadrenal pheochromocytomas arise in the abdomen (Figure 10-13); a few are found in the chest or neck. The tumor may be located anywhere along the sympathetic nervous system, in the organ of Zuckerkandl, and in chemoreceptor tissues such as the carotid body and the glomus jugulare, in the wall of the urinary bladder, or even in the kidney or ureter. Masses may displace the ureter or kidney or may appear as filling defects in the bladder.

MRI can demonstrate the relationship of the tumor to surrounding structures in the coronal plane (Figure 10-14) and has a higher sensitivity than CT. On T2-weighted spin-echo images, the extreme hyperintensity of the tumor (because of its water content) makes it stand out from the surrounding structures. When CT and MRI findings are inconclusive, a radionuclide scan using metaiodobenzylguanidine is highly sensitive for localizing ectopic pheochromocytomas, but this agent is not readily available.

In patients with pheochromocytomas, the arterial injection of contrast material causes a sharp elevation in blood pressure, which must be controlled by α-adrenergic blocking agents. Therefore, arteriography is hazardous in these patients.


Neuroblastoma, a tumor of adrenal medullary origin, is the second most common malignancy in children. About 10% of these tumors arise outside the adrenal gland, primarily in sympathetic ganglia in the neck, chest, abdomen, or pelvis. The tumor is highly malignant and tends to attain great size before its detection.

Radiographic Appearance: Calcification is common in neuroblastoma (occurring in about 50% of cases), in contrast to the relatively infrequent calcification in Wilms’ tumor, from which neuroblastoma must be differentiated. Calcification in a neuroblastoma has a fine granular or stippled appearance (Figure 10-15A). Occasionally, there may be a single mass of amorphous calcification. Calcification can also develop in metastases of neuroblastoma in the paravertebral lymph nodes and the liver.

Intravenous urography usually demonstrates downward and lateral renal displacement by the tumor mass (Figure 10-16). Neuroblastoma tends to cause the entire kidney and its collecting system to be displaced as a unit, unlike Wilms’ tumor, which has an intrarenal origin and thus tends to distort and widen the pelvicalyceal system.

Because of its nonionizing character, ultrasound is a superb modality for evaluating abdominal masses in children. A neuroblastoma appears as a solid or semisolid mass that is separate from the kidney. It appears as a poorly defined heterogenic mass, unlike the well-defined and relatively homogeneous Wilms’ tumor. A neuroblastoma is often diffusely hyperechogenic, probably because of necrosis, calcification, and hemorrhage. On contrast-enhanced CT, necrosis and hemorrhage cause the tumor to appear heterogeneous and often lobulated. Because CT can demonstrate evidence of tumor spread to lymph nodes and the sympathetic chain, widening of the paravertebral stripe (also seen on plain images) (Figure 10-17), and metastases to the liver and chest, it has become the most commonly used imaging study to diagnose neuroblastomas. This modality is also used to assess the response to treatment. MRI, which like ultrasound does not use ionizing radiation, may offer a safer approach to demonstrating characteristics typical of tumor tissue. Neuroblastomas usually produce a hypointense signal on T1-weighted images and are hyperintense on T2-weighted images. On contrast-enhanced T1-weighted images, hemorrhage appears as a hyperintense signal.

Apr 10, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Endocrine System
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