SIADH

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of impaired water excretion due to the inability to suppress the secretion of antidiuretic hormone (ADH). There are many causes of SIADH, including increased pituitary production of ADH (related to trauma, disease, or medications), ectopic production of ADH by a tumor, and hereditary disorders. SIADH is characterized by impaired water excretion, leading to dilutional hyponatremia. This condition is usually asymptomatic but may cause neurologic symptoms. SIADH should be suspected in any patient with hyponatremia, hypo-osmolality, and high urine osmolality.

Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. The work of hormones allows the body to maintain homeostasis and regulate growth and development. Hormones are typically either made from amino acids or derived from cholesterol (the latter group being known as steroid hormones). Hormones exert their effects by binding to receptors either on the cell surface (most amino acid–based hormones) or within the cytosol (steroid hormones). Ultimately, binding to receptors triggers changes in gene expression or enzymatic activity within the cell.

The pituitary gland, also known as the hypophysis, is considered the “master endocrine gland” because it releases hormones that regulate the activity of multiple major endocrine organs in the body. The gland sits on the sella turcica, just below the hypothalamus, which is the primary regulator of the pituitary gland. The pituitary gland has 2 lobes: the anterior lobe (the adenohypophysis) and the posterior lobe (the neurohypophysis). Each lobe has its own regulation, set of secretory products, and feedback loops. Abnormalities in the pituitary gland can lead to a wide range of clinical conditions, some of which include hyperprolactinemia, acromegaly, hyperthyroidism or hypothyroidism, and central diabetes insipidus.

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of impaired water excretion due to the inability to suppress the secretion of antidiuretic hormone (ADH). The inappropriate secretion of ADH can be due to various causes, including increased production by the pituitary gland due to trauma, disease, or certain medications; the ectopic secretion of ADH by cancer; or hereditary causes (nephrogenic SIADH). Syndrome of inappropriate antidiuretic hormone secretion is characterized by impaired water excretion leading to dilutional hyponatremia, which is mainly asymptomatic but may cause neurologic symptoms. Syndrome of inappropriate antidiuretic hormone secretion should be suspected in any patient with hyponatremia, hypo-osmolality, and high urine osmolality.