Disorders of the Neurohypophysis¶

Chapter 393 | Harrison's 22e · Part 12 – Endocrinology & Metabolism · Chapter 393

Key Clinical Points¶

AVP (Antidiuretic Hormone) is a nonapeptide synthesized in the hypothalamus and released from the posterior pituitary to concentrate urine.
AVP secretion is primarily regulated by 'effective' osmotic pressure (concentration of sodium and its anions).
A deficiency in AVP leads to polyuria and polydipsia; an excess of AVP leads to hyponatremia.
Copeptin serves as a highly accurate biomarker (97% accuracy) for distinguishing AVP deficiency from primary polydipsia at high sodium levels (ge 150 mmol/L).
Nausea is an extremely potent stimulus for AVP release, capable of causing 50- to 100-fold increases in plasma AVP even if transient and without vomiting.
The differentiation between central and nephrogenic DI relies on the response to desmopressin (2 μg) and copeptin measurements.
Adipsic AVP deficiency is characterized by a lack of both AVP and the corresponding thirst response.
TSH-secreting adenomas are identified by elevated free T levels, high α subunits, and pituitary mass; SRL treatment can shrink tumor mass in 50% of patients and improve visual fields in 75%.

DEFINITION & CLASSIFICATION¶

• Neurohypophysis: Posterior pituitary; consists of the distal axons of the hypothalamic magnocellular neurons. • Location: Cell bodies are located in paired paraventricular and supraoptic nuclei of the hypothalamus. • Arginine Vasopressin (AVP): Also known as antidiuretic hormone (ADH). ◦ Structure: Nonapeptide composed of a six-member disulfide ring and a tripeptide tail. ◦ Function: Acts on renal tubules to reduce water loss by concentrating urine. • Oxytocin: Produced in the same neurons; stimulates postpartum milk letdown and elicits socioemotional responses. • AVP Deficiency/Resistance: Causes a syndrome of large amounts of dilute urine (polyuria). • Excess AVP: Impairs urinary water excretion and predisposes to hyponatremia.

ETIOLOGY & PATHOPHYSIOLOGY¶

• AVP Synthesis & Secretion: ◦ Precursor: Polypeptide including AVP, neurophysin, and copeptin (all encoded by a single gene on chromosome 20). ◦ Processing: Folded/processed in neurosecretory vesicles; transported down axons to the posterior pituitary for release via exocytosis. • Regulation of AVP: ◦ Primary Regulator: "Effective" osmotic pressure (determined by concentration of sodium and its anions). ◦ Osmoreceptors: Located in anteromedial hypothalamus; highly sensitive to small changes in plasma sodium/anions. ◦ Hemodynamic Influence: Baroregulation via heart/large artery receptors → Vagus/Glossopharyngeal nerves → Brainstem → Hypothalamus. ◦ Other Stimuli: Nausea (extremely potent, 50- to 100-fold increase), acute hypoglycemia, glucocorticoid deficiency, smoking, and potentially angiotensin. • Mechanism of Action: ◦ Binding: AVP binds to V_2 receptors on the basolateral membrane. ◦ Signaling: Increases intracellular cyclic AMP → translocation of Aquaporin-2 (AQP 2) to the apical membrane. ◦ Result: Increased permeability allows water influx from lumen into cell → exit via AQP 3 and AQP 4 on basolateral side → concentrated urine.

CLINICAL FEATURES¶

• AVP Deficiency/Resistance: Production of large amounts of dilute urine. • Excess AVP: Impaired urinary water excretion → hyponatremia. • Adipsic AVP Deficiency: A specific condition where AVP is deficient without a corresponding thirst response. • TSH-Secreting Adenomas (Clinical Context): ◦ Diagnosis: Elevated serum free T levels, inappropriately normal or high TSH secretion, and MRI evidence of a pituitary mass. ◦ Biomarkers: Elevated glycoprotein hormone α subunits are seen in many patients. ◦ Exclusions: Rule out resistance to thyroid hormone (autosomal dominant; mutation in β receptor) and dysalbuminemic hyperthyroxinemia syndromes (mutation in binding proteins). ◦ Treatment: Surgery (transsphenoidal), thyroid ablation, or antithyroid drugs (methimazole, propylthiouracil). ◦ SRL Therapy: Effectively normalizes TSH and α subunit hypersecretion, shrinks tumor mass in 50% of patients, and improves visual fields in 75% of patients.

DIAGNOSTIC APPROACH¶

Initial Assessment: Identify symptoms of polyuria, nocturia, and/or persistent thirst in the absence of glucosuria.
24-hour Urine Collection: Collect urine on unrestricted fluid intake. → If Volume >50 mL/kg per day AND Urine Osmolality <800 mOsm/kg → Proceed to serum sodium measurement.
Serum Sodium Measurement: → If <135 mmol/L → Primary Polydipsia (patient drinks excess water). → If 136-146 mmol/L → Perform Water Deprivation Test. → If Urine Osmolality 300-800 mOsm/kg → Desmopressin test (2 μg) → If increase >9% → Partial Central DI. → If Urine Osmolality <300 mOsm/kg → Desmopressin test (2 μg) → → If increase <50% → Nephrogenic DI. → If increase >50% → Complete Central or Nephrogenic DI. → If >147 mmol/L → Copeptin-based diagnosis (if available). → Baseline Copeptin: → If ge 21.4 pmol/L → AVP Resistance. → Hypertonic Saline Test (at serum sodium ge 150 mmol/L) → Copeptin measurement: → If >4.9 pmol/L → AVP Deficiency (Accuracy: 97%). → If le 4.9 pmol/L → Primary Polydipsia.

MANAGEMENT & TREATMENT¶

Initial Treatment for Adipsic AVP Deficiency: → Replace AVP with sufficient vasopressin (DDAVP).
Inpatient Management: → Monitor fluid input/output initially to achieve eunatremia.
Weight Management: → Record patient's eunatremic weight. → Weigh daily. → If weight falls below eunatremic level → Replace with equivalent volume of fluid to restore eunatremic weight.
Outpatient Recommendations: → Recommend 1.5–2 L of fluid intake per day (assuming lower urinary losses). → Recommend increased fluid intake in times of increased perspiration or high ambient temperatures.
Monitoring: → Regular plasma sodium measurements.

KEY PEARLS & HIGH-YIELD POINTS¶

• Copeptin Accuracy: Copeptin measurement at high sodium levels (ge 150 mmol/L) has an overall diagnostic accuracy of 97% to distinguish AVP deficiency from primary polydipsia. • Nausea Potency: Nausea is an extremely potent stimulus for AVP release; it can cause 50- to 100-fold increases in plasma AVP even if transient and without vomiting. • Differentiation Logic: The distinction between 'Partial' and 'Complete' central DI depends on the urine osmolality response to water deprivation and subsequent desmopressin challenge. • AVP Synthesis: A single gene on chromosome 20 encodes a precursor that is processed into AVP, neurophysin, and copeptin.

Reference Tables¶

TABLE 393-1 Etiology of Polyuria–Polydipsia Syndromes¶

Harrison's 22e, p.3017

BASIC DEFECT	ACQUIRED CAUSES	HEREDITARY CAUSES
AVP Deficiency
Deficiency in AVP synthesis or secretion	• Trauma (surgery, deceleration injury) • Neoplasia (craniopharyngioma, meningioma, germinoma, metastases) • Vascular (cerebral or hypothalamic hemorrhage, infarction or ligation of anterior communicating artery aneurysm) • Granulomatous (histiocytosis, sarcoidosis) • Infectious (meningitis, encephalitis, tuberculosis) • Inflammatory or autoimmune (lymphocytic infundibuloneurohypophysitis, IgG4 neurohypophysitis) • Drug or toxin exposure • Osmoreceptor dysfunction (adipsic DI) • Others (hydrocephalus, ventricular or suprasellar cyst, trauma, and degenerative diseases) • Idiopathic	• Autosomal dominant: AVP mutations • Autosomal recessive, type a and b: AVP mutations • Autosomal recessive, type c: WFS1 mutations • Autosomal recessive, type d: PCSK1 mutations • X-linked recessive: gene unknown
AVP Resistance
Primary Polydipsia
Excessive fluid intake at a diminished set point	• Dipsogenica (idiopathic or similar lesions as with central DI) • Psychosis intermittent hyponatremia–polydipsia (PIP) syndrome • Compulsive water drinking • Health enthusiasts	NA
Gestational AVP Deficiency

TABLE 393-2 Approach to the Management of Water Balance for Patients with Adipsic Arginine Vasopressin (AVP) Deficiency…¶

Harrison's 22e, p.3019

1. Replace AVP with sufficient vasopressin (DDAVP).
  2. Monitor fluid input/output initially as an inpatient to achieve eunatremia.
  3. Weigh and record patient’s eunatremic weight.
  4. Recommend 1.5–2 L of fluid intake per day assuming urinary losses are less.
  5. Weigh daily.
  6. If below eunatremic weight, then replace with equivalent volume of fluid to
  restore eunatremic weight.
  7. Recommend increased fluid intake in times of increased perspiration or
  ambient temperatures.
  8. Regular plasma sodium measurements.