[4] Cases in which the urine is dilute include adrenal insufficiency, hypothyroidism, and drinking too much water or too much beer.

[4] Cases in which the urine is concentrated include syndrome of inappropriate antidiuretic hormone secretion (SIADH).

However, mild hyponatremia (plasma sodium levels at 131–135 mmol/L) may be associated with complications and subtle symptoms[14] (for example, increased falls, altered posture and gait, reduced attention, impaired cognition, and possibly higher rates of death).

[1] When sodium levels in the blood become very low, water enters the brain cells and causes them to swell (cerebral edema).

This can lead to headache, nausea, vomiting, confusion, seizures, brain stem compression and respiratory arrest, and non-cardiogenic accumulation of fluid in the lungs.

A gradual drop, even to very low levels, may be tolerated well if it occurs over several days or weeks, because of neuronal adaptation.

[25] Available evidence suggests that all classes of psychotropics, i.e., antidepressants, antipsychotics, mood stabilizers, and sedative/hypnotics can lead to hyponatremia.

[26] Miscellaneous causes that are not included under the above classification scheme include the following: The causes of and treatments for hyponatremia can only be understood by having a grasp of the size of the body fluid compartments and subcompartments and their regulation; how under normal circumstances the body is able to maintain the sodium concentration within a narrow range (homeostasis of body fluid osmolality); conditions can cause that feedback system to malfunction (pathophysiology); and the consequences of the malfunction of that system on the size and solute concentration of the fluid compartments.

[27] There is a hypothalamic-kidney feedback system which normally maintains the concentration of the serum sodium within a narrow range.

This system operates as follows: in some of the cells of the hypothalamus, there are osmoreceptors which respond to an elevated serum sodium in body fluids by signalling the posterior pituitary gland to secrete antidiuretic hormone (ADH) (vasopressin).

[citation needed] Hyponatremia occurs in one of two ways: either the osmoreceptor-aquaporin feedback loop is overwhelmed, or it is interrupted.

"Impairment of urine diluting ability related to ADH" occurs in nine situations: 1) arterial volume depletion 2) hemodynamically mediated, 3) congestive heart failure, 4) cirrhosis, 5) nephrosis, 6) spinal cord disease, 7) Addison's disease, 8) cerebral salt wasting, and 9) syndrome of inappropriate antidiuretic hormone secretion (SIADH).

Cell swelling also produces activation of volume-regulated anion channels which is related to the release of taurine and glutamate from astrocytes.

[33] The history, physical exam, and laboratory testing are required to determine the underlying cause of hyponatremia.

In isotonic hyponatremia a measurement error due to high blood triglyceride level (most common) or paraproteinemia occurs.

[16] Sodium deficit = (140 – serum sodium) × total body water[5] Total body water = kilograms of body weight × 0.6 Options include: In persons with hyponatremia due to low blood volume (hypovolemia) from diuretics with simultaneous low blood potassium levels, correction of the low potassium level can assist with correction of hyponatremia.

[37] In the United States they are recommended in those with SIADH, cirrhosis, or heart failure who fail limiting fluid intake.

[12] They may also be used in people with chronic hyponatremia due to SIADH that is insufficiently responsive to fluid restriction and/or sodium tablets.

[16] Demeclocycline, while sometimes used for SIADH, has significant side effects including potential kidney problems and sun sensitivity.

Hyponatremia has been reported in up to 30% of the elderly in nursing homes and is also present in approximately 30% of people who are depressed on selective serotonin reuptake inhibitors.