The rate of sodium correction in hypernatraemia is the most clinically critical parameter and the source of the most serious iatrogenic complications — the brain adapts to chronic hypernatraemia by generating idiogenic osmoles (taurine, glutamine, inositol, myoinositol) within neurons to prevent cell shrinkage. If hypernatraemia is corrected too rapidly, the intracellular osmoles cannot be cleared quickly enough, causing net influx of water into neurons — cerebral oedema, resulting in seizures, herniation, and death. The correction rate rule: for chronic hypernatraemia (duration >48 hours or unknown): maximum fall in serum sodium of about 0.5 mmol/L per hour and maximum 10-12 mmol/L per 24 hours, usually replacing the deficit over about 48 hours with adjustment for ongoing losses and comorbidity. For clearly acute hypernatraemia with neurological symptoms: a faster initial reduction (about 5 mmol/L in the first hour) may be needed under senior critical-care/endocrine supervision, but the fall should not then exceed the agreed 24-hour limit (Society for Endocrinology: 10 mmol/L in 24 hours). In practice, most community-presenting hypernatraemia has been developing over days — treat as chronic. Serial 2-4 hourly sodium monitoring during correction is mandatory.

Sodium interpretation in hyperglycaemia needs care when reviewing a patient with both elevated glucose and elevated or apparently normal sodium — hyperglycaemia causes water to move from the intracellular to extracellular compartment (by osmotic effect), diluting the plasma sodium. As a result, the measured sodium appears lower than the true sodium that would exist if glucose were normal. Corrected-sodium coefficients vary according to the formula and glucose range, so no single coefficient is universally correct. In HHS, management is guided primarily by clinical volume status, measured sodium, the calculated/measured osmolality trend, the glucose fall, urine output and renal and cardiac status. JBDS emphasises that sodium may initially rise as glucose falls with treatment (water moves back intracellularly), and that this alone does not indicate a need to switch to hypotonic fluid. Do not switch fluids solely because sodium rises while glucose is falling — monitor calculated osmolality and fluid balance and use the local JBDS pathway.

The urine osmolality in hypernatraemia is the key discriminating investigation between appropriate physiological response, diabetes insipidus, and primary polydipsia — in normal hypernatraemia from water depletion, the hypothalamus detects high plasma osmolality and releases ADH, which causes the kidney to retain water and concentrate the urine maximally. Expected urine osmolality in appropriate response to hypernatraemia: >600-800 mOsm/kg (very concentrated urine). In diabetes insipidus: the kidney cannot concentrate urine because either ADH is absent (central DI) or the kidney cannot respond to ADH (nephrogenic DI). Urine osmolality in DI is inappropriately dilute for the degree of hypernatraemia — but urine osmolality alone does not distinguish central DI, nephrogenic DI and primary polydipsia; interpret it with serum osmolality, urine volume, renal function and medication history. The diagnostic test is specialist-supervised water deprivation (or copeptin-based) testing in an endocrine setting, never arranged directly from primary care in a hypernatraemic patient — urine osmolality is measured as fluid is withheld, then desmopressin is given. If urine concentrates after desmopressin: central DI. If urine does not concentrate: nephrogenic DI. If urine concentrates before desmopressin: not DI (appropriate response, or primary polydipsia).

Lithium-induced nephrogenic DI is one of the most common and practically important causes of polyuria encountered in primary care — a substantial proportion of patients on long-term lithium develop some degree of nephrogenic DI, ranging from mild polyuria to severe diabetes insipidus with very large urine volumes. The mechanism: lithium enters renal collecting duct principal cells via the epithelial sodium channel (ENaC) and inhibits the adenylate cyclase pathway, reducing aquaporin-2 (AQP2) water channel expression and impairing ADH-mediated water reabsorption. In early stages, this is reversible on stopping lithium; in patients with prolonged lithium treatment and high trough levels, some tubular damage may be permanent. In a patient who needs to continue lithium, amiloride (blocks ENaC, reducing lithium entry into principal cells), lithium dose reduction or switching are specialist decisions with psychiatry, endocrinology or nephrology; adequate hydration matters because dehydration increases lithium serum levels and toxicity risk.

Oral rehydration may be appropriate for a mild, isolated hypernatraemia (sodium 146–149 mmol/L) in a clinically well, cognitively intact adult who can drink, once volume status, renal function, glucose, medications and the cause have been reviewed — intravenous fluids introduce the risk of overcorrection and are not necessary in mild cases with adequate oral access. Free water (plain water, not sports drinks or normal saline) is the appropriate correction fluid for water-deficit hypernatraemia. Oral rehydration solution (ORS) may be used for combined isotonic + water losses (gastroenteritis). The important contraindication to oral correction: any patient with reduced consciousness, swallowing difficulty, or who cannot reliably drink adequate volumes needs IV correction in a monitored setting. Arrange an urgent repeat at an interval set by the local laboratory and clinical context; the fall should not exceed about 0.5 mmol/L per hour or 10–12 mmol/L in 24 hours.

The desmopressin hyponatraemia risk is a critical prescribing safety point that must be communicated to every patient on desmopressin for central DI — desmopressin is a synthetic analogue of ADH that maximally stimulates V2 receptors in the renal collecting duct, causing maximal urinary concentration and water retention. If the patient drinks ad libitum with unrestricted fluid intake while on desmopressin, they will over-retain water and develop hyponatraemia. The safety instruction: patients on desmopressin must: (1) take the lowest effective dose; (2) not over-drink; (3) follow their individualised written plan from endocrinology, including how to recognise breakthrough polyuria and whether any dose delay is permitted; and (4) have serum sodium checked after any dose change, illness, or change in fluid intake. Hyponatraemia from desmopressin overdose in central DI patients presents with headache, nausea, seizures, and confusion — identical to SIADH — and is treated by stopping desmopressin and fluid restriction, with specialist advice. Conversely, vomiting, nil-by-mouth status, impaired consciousness and inability to access water are emergencies — omitted desmopressin or impaired water access can cause life-threatening hypernatraemia.

The JBDS HHS management protocol represents current best practice for a condition with historically high mortality — the key departure from classical teaching is that insulin infusion should be DELAYED in HHS until adequate fluid resuscitation is underway and the glucose is no longer falling with fluids alone (unless there is significant ketonaemia). The reason: in HHS, the high glucose is maintaining intravascular osmolality and volume — rapid insulin administration before adequate fluid replacement causes glucose (and therefore osmolality) to fall precipitously, drawing water back intracellularly and causing circulatory collapse. Equally important: the initial fluid of choice for HHS is 0.9% NaCl despite the hypernatraemia — the priority is restoring circulating volume and avoiding further osmotic instability. The switch to 0.45% NaCl is made only if the osmolality is no longer falling despite adequate 0.9% NaCl replacement. This is counterintuitive for trainees who naturally want to give hypotonic fluid for hypernatraemia — understanding the two-phase HHS protocol prevents this management error.

The dehydration assessment in elderly care home residents is one of the most common and practically impactful primary care interventions — water-loss dehydration is common in care home residents (about 1 in 5 in the UK DRIE study) and common clinical signs do not reliably detect it. The consequences: impaired cognition, increased falls risk, constipation, urinary tract infection susceptibility, pressure sore development, and acute kidney injury vulnerability. CQC regulation 14 (nutrition and hydration) provide structured approaches for care homes. GPs conducting care home visits or reviewing care home residents should check: whether the resident has a documented daily fluid intake target, whether fluid intake is monitored, whether thickened fluids are offered where needed for dysphagia, and whether medications that increase water loss (diuretics, laxatives) are being managed in the context of adequate fluid replacement.

The central DI triphasic response after pituitary or hypothalamic surgery is a specific clinical pattern that GPs whose patients have had trans-sphenoidal pituitary surgery must know — Phase 1 (days 1-3): acute DI from surgical trauma and oedema disrupting ADH release from the neurohypophysis — polyuria + hypernatraemia requires desmopressin. Phase 2 (days 4-7): SIADH — damaged neurons release stored ADH uncontrolled — water retention + hyponatraemia. Stop desmopressin during this phase. Phase 3 (permanent): if the ADH-producing neurons are permanently damaged, permanent DI — restart desmopressin. If neurons recover, DI resolves. The risk of unrecognised Phase 2 SIADH while continuing desmopressin is severe hyponatraemia. Post-operative endocrine monitoring is specialist-led but GPs should be aware of this pattern when patients present post-operatively with sodium abnormalities.