What is the appropriate diagnosis and management for a patient with hyposecretion of ADH (Antidiuretic Hormone), low urine osmolality, and normal serum sodium levels?

Diagnosis: Central (Neurogenic) Diabetes Insipidus

This patient has central diabetes insipidus, confirmed by the combination of low ADH (<0.8 pg/mL), inappropriately dilute urine (osmolality 220 mOsm/kg) in the setting of high-normal serum osmolality (295 mOsm/kg), and normal serum sodium (143 mEq/L). 1

Diagnostic Interpretation

• The urine osmolality of 220 mOsm/kg is pathologically dilute given the serum osmolality of 295 mOsm/kg—normal kidneys would concentrate urine to >600 mOsm/kg under these conditions, confirming the kidneys' inability to respond appropriately to osmotic stress 1, 2

• The low ADH level (<0.8 pg/mL) with high-normal serum osmolality (295 mOsm/kg) is diagnostic of central diabetes insipidus, as ADH should be elevated in response to this degree of serum hyperosmolality 1, 3

• The normal serum sodium (143 mEq/L) indicates the patient is maintaining adequate fluid intake through their intact thirst mechanism, which is typical for patients with free access to water 1, 2

• The combination of urine osmolality <300 mOsm/kg with serum osmolality ≥295 mOsm/kg confirms diabetes insipidus, distinguishing this from primary polydipsia where both values would be low 1

Required Additional Workup

Before initiating treatment, obtain the following tests:

• Plasma copeptin level to definitively distinguish central DI (copeptin <21.4 pmol/L) from nephrogenic DI (copeptin >21.4 pmol/L), though the low ADH already strongly suggests central DI 1, 4

• 24-hour urine volume to quantify polyuria (expected >3 L/day in adults or >4 mL/kg/hr in children) 1, 4

• MRI of the sella with dedicated pituitary sequences to identify structural causes, as approximately 50% of central DI cases have identifiable etiologies including tumors, infiltrative diseases, inflammatory processes, or post-surgical/traumatic changes 1

• Serum creatinine, electrolytes (including calcium), and glucose to rule out other causes of polyuria and assess baseline renal function 1

Treatment Approach

Desmopressin (synthetic ADH) is the treatment of choice for central diabetes insipidus:

• Start with desmopressin 2-4 mcg subcutaneously or intravenously in divided doses, or use intranasal formulation 10-20 mcg/day in 1-2 divided doses, or oral formulation 0.1-0.2 mg twice daily 1, 5, 6

• The intranasal route may be compromised in situations including nasal congestion, nasal discharge, atrophy of nasal mucosa, severe atrophic rhinitis, impaired consciousness, or post-cranial surgery with nasal packing 5

• Allow the patient to determine fluid intake based on thirst rather than prescribing specific amounts, as their osmosensors are typically more sensitive and accurate than any medical calculation 1, 2

• Check serum sodium within 7 days and at 1 month after starting treatment, then periodically, as hyponatremia is the main complication of desmopressin therapy 1

Critical Management Principles

Ensure free access to plain water or hypotonic fluids at all times:

• Never restrict water access—this is a life-threatening error that leads to severe hypernatremic dehydration 1, 2

• If IV hydration is needed, use 5% dextrose in water (hypotonic fluid), NOT normal saline or electrolyte solutions, to avoid worsening hypernatremia 1, 4

• Patients with true diabetes insipidus must drink enormous volumes just to stay alive—the polyuria is not from excessive drinking but from the kidneys' inability to retain water 2

Common Pitfalls to Avoid

• Do not confuse this with SIADH, which presents with hyponatremia, low serum osmolality, and inappropriately high urine osmolality—the exact opposite picture 1

• Do not confuse with diabetes mellitus, which causes polyuria through osmotic diuresis from glucosuria and would show elevated blood glucose, not low ADH 1

• Do not use electrolyte-containing solutions (like Pedialyte or normal saline) for routine hydration, as these contain substantial sodium loads that can worsen the condition 1

• Some patients may show decreased responsiveness or shortened duration of effect after >6 months of treatment, which may be due to local inactivation of the peptide rather than antibody development 5

Follow-Up Monitoring

Establish regular follow-up based on age and stability:

• Adults should be seen every 6-12 months with clinical assessment including weight, polyuria symptoms, and biochemical work-up (serum sodium, potassium, chloride, bicarbonate, creatinine, uric acid) 1

• Perform renal ultrasound every 2 years to monitor for urinary tract dilatation or bladder dysfunction from chronic polyuria, as approximately 46% of patients develop urological complications 1

• Annual urinalysis including osmolality and 24-hour urine volume measurement to assess treatment adequacy 1