Endocrine and Metabolic Disorders

The endocrine system coordinates functioning between different organs through hormones, which are chemicals released into the bloodstream from specific types of cells within endocrine (ductless) glands. Once in circulation, hormones affect function of the target tissues, which may be another endocrine gland or an end organ. Some hormones exert an effect on cells of the organ from which they were released (paracrine effect), some even on the same cell type (autocrine effect).

Metabolic processes continually produce acid and, to a lesser degree, base. Hydrogen ion (H+) is especially reactive; it can attach to negatively charged proteins and, in high concentrations, alter their overall charge, configuration, and function. To maintain cellular function, the body has elaborate mechanisms that maintain blood H+ concentration within a narrow range—typically 37 to 43 nEq/L [37 to 43 nmol/L] (pH 7.43 to 7.37, where pH =−log [H+]) and ideally 40 nEq/L [40 nmol/L] (pH = 7.40). Disturbances of these mechanisms can have serious clinical consequences.

The adrenal glands, located on the cephalad portion of each kidney (see figure Adrenal glands), consist of a

Amyloidosis is any of a group of disparate conditions characterized by extracellular deposition of insoluble fibrils composed of misaggregated proteins. These proteins may accumulate locally, causing relatively few symptoms, or widely, involving multiple organs and causing severe multiorgan failure. Amyloidosis can occur de novo or be secondary to various infectious, inflammatory, or malignant conditions. Diagnosis is by biopsy of affected tissue; the amyloidogenic protein is typed using a variety of immunohistologic and biochemical techniques. Treatment varies with the type of amyloidosis.

Carcinoid tumors develop from neuroendocrine cells in the gastrointestinal tract (90%), pancreas, pulmonary bronchi, and rarely the genitourinary tract. More than 95% of all gastrointestinal carcinoids originate in only 3 sites: the appendix, ileum, and rectum.

Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia, polyphagia, polyuria, and blurred vision. Later complications include vascular disease, peripheral neuropathy, nephropathy, and predisposition to infection. Diagnosis is by measuring plasma glucose. Treatment is diet, exercise, and drugs that reduce glucose levels, including insulin, oral antihyperglycemic drugs, and non-insulin injectable drugs. Complications can be delayed or prevented with adequate glycemic control; heart disease remains the leading cause of mortality in diabetes mellitus.

Hypernatremia is a serum sodium concentration > 145 mEq/L (> 145 mmol/L). It implies a deficit of total body water relative to total body sodium caused by water intake being less than water losses. A major symptom is thirst; other clinical manifestations are primarily neurologic (due to an osmotic shift of water out of brain cells), including confusion, neuromuscular excitability, seizures, and coma. Diagnosis requires measurement of serum sodium and sometimes other laboratory tests. Treatment is usually controlled water replacement. When the response to treatment is poor, testing (eg, monitored water deprivation or administration of vasopressin) is directed at detecting causes other than decreased water intake.

Body fluid volume and electrolyte concentration are normally maintained within very narrow limits despite wide variations in dietary intake, metabolic activity, and environmental stresses. Homeostasis of body fluids is preserved primarily by the kidneys.

Lipids are fats that are either absorbed from food or synthesized by the liver. Triglycerides (TGs) and cholesterol contribute most to disease, although all lipids are physiologically important.

The multiple endocrine neoplasia (MEN) syndromes comprise 3 genetically distinct familial diseases involving adenomatous hyperplasia and malignant tumors in several endocrine glands.

There are typically four parathyroid glands. As the name implies, they are located near the thyroid gland, although the number and particularly the location is quite variable. These pea-sized glands play a vital role in maintaining the body's calcium level. (See also Hypercalcemia and Hypocalcemia.)

The pituitary gland controls the functions of peripheral endocrine glands (see figure The pituitary and its target organs). Pituitary structure and function and relationships between the hypothalamus and the pituitary gland are discussed in Overview of the Endocrine System.

Polyglandular deficiency syndromes (PDS) are characterized by sequential or simultaneous deficiencies in the function of several endocrine glands that have a common cause. Etiology is most often autoimmune. Categorization depends on the combination of deficiencies, which fall within 1 of 3 types. Diagnosis requires measurement of hormone levels and autoantibodies against affected endocrine glands. Treatment includes replacement of missing or deficient hormones and sometimes immunosuppressants.

Porphyrias are rare disorders in which hemoglobin is abnormally metabolized due to genetic or acquired deficiencies of enzymes of the heme biosynthetic pathway. These deficiencies allow heme precursors to accumulate, causing toxicity. Porphyrias are defined by the specific enzyme deficiency. Two major clinical manifestations occur: neurovisceral abnormalities (the acute porphyrias) and cutaneous photosensitivity (the cutaneous porphyrias).

The thyroid gland, located in the anterior neck just below the cricoid cartilage, consists of 2 lobes connected by an isthmus. Follicular cells in the gland produce the 2 main thyroid hormones: