NR 507 WEEK 6 MASTER QUESTIONS AND ANSWERS: ENDOCRINE SYSTEM
ALTERATIONS
ADVANCED PATHOPHYSIOLOGY FOCUS: HYPOTHALAMUS, PITUITARY,
ADRENAL, AND DIABETES
SECTION ONE: HYPOTHALAMUS AND PITUITARY GLAND ALTERATIONS
Question 1
A patient is diagnosed with Syndrome of Inappropriate Antidiuretic Hormone (SIADH).
Explain the precise cellular mechanism by which excessive Antidiuretic Hormone (ADH)
causes the patient's severe hyponatremia and decreased serum osmolality, while the urine
osmolality remains inappropriately high.
Answer:
ADH (vasopressin) is secreted by the posterior pituitary and acts on the V2 receptors of the
collecting ducts in the kidneys. Under normal conditions, these ducts are impermeable to
water. When ADH binds to these receptors, it triggers an intracellular cyclic AMP cascade
that causes the insertion of aquaporin-2 water channels into the luminal membrane of the
collecting ducts. In SIADH, there is non-physiologic, excessive secretion of ADH. This causes
massive reabsorption of free water out of the tubular lumen and back into the bloodstream,
independent of the body's serum osmolality status. Because this free water is being
reabsorbed without corresponding electrolytes (like sodium), it dilutes the serum sodium,
causing hyponatremia. It also dilutes the serum, leading to decreased serum osmolality.
Because the aquaporin channels are actively pulling water out of the tubule, the remaining
urine becomes highly concentrated, resulting in an inappropriately high urine osmolality
(greater than 100 mOsm/kg) despite the serum being dilute.
, Question 2
A patient presents with central diabetes insipidus following a traumatic brain injury.
Contrast the pathophysiologic mechanism of central diabetes insipidus with nephrogenic
diabetes insipidus. Why does administering Desmopressin (a synthetic ADH analog) correct
the polyuria in central diabetes insipidus but has no effect on nephrogenic diabetes
insipidus?
Answer:
Central diabetes insipidus is caused by a deficiency in ADH production or release. This
usually occurs due to damage to the hypothalamic neurons (supraoptic and paraventricular
nuclei) or the posterior pituitary stalk from trauma, tumors, or surgery. Without ADH, the
collecting ducts remain impermeable to water. Large volumes of dilute fluid are excreted,
leading to polyuria and hypernatremia. Because the end organ (the kidney) is intact,
administering Desmopressin replaces the missing hormone, inserts the aquaporin
channels, and stops the polyuria. Nephrogenic diabetes insipidus is caused by an end-organ
resistance to ADH. The posterior pituitary is secreting normal or high levels of ADH, but the
kidneys cannot respond. This is typically due to genetic mutations affecting the V2
receptors or acquired defects (like lithium toxicity) that destroy the aquaporin channels.
Because the defect is at the receptor or channel level, adding more hormone
(Desmopressin) will have zero therapeutic effect.
Question 3
A 42-year-old female presents with galactorrhea, amenorrhea, and visual field defects. MRI
reveals a 1.8 cm pituitary macroadenoma. What specific anatomical structure is being
compressed to cause the visual field defect, and what is the classic visual field loss
observed? Furthermore, explain the pathophysiologic mechanism by which this specific
tumor causes amenorrhea.
Answer:
The pituitary gland sits in the sella turcica, directly inferior to the optic chiasm. As a
pituitary macroadenoma grows upward, it compresses the optic chiasm. The optic chiasm
contains the crossing fibers of the nasal retinas, which carry visual information from the
temporal (lateral) visual fields. Compression of these crossing fibers causes a bilateral loss
ALTERATIONS
ADVANCED PATHOPHYSIOLOGY FOCUS: HYPOTHALAMUS, PITUITARY,
ADRENAL, AND DIABETES
SECTION ONE: HYPOTHALAMUS AND PITUITARY GLAND ALTERATIONS
Question 1
A patient is diagnosed with Syndrome of Inappropriate Antidiuretic Hormone (SIADH).
Explain the precise cellular mechanism by which excessive Antidiuretic Hormone (ADH)
causes the patient's severe hyponatremia and decreased serum osmolality, while the urine
osmolality remains inappropriately high.
Answer:
ADH (vasopressin) is secreted by the posterior pituitary and acts on the V2 receptors of the
collecting ducts in the kidneys. Under normal conditions, these ducts are impermeable to
water. When ADH binds to these receptors, it triggers an intracellular cyclic AMP cascade
that causes the insertion of aquaporin-2 water channels into the luminal membrane of the
collecting ducts. In SIADH, there is non-physiologic, excessive secretion of ADH. This causes
massive reabsorption of free water out of the tubular lumen and back into the bloodstream,
independent of the body's serum osmolality status. Because this free water is being
reabsorbed without corresponding electrolytes (like sodium), it dilutes the serum sodium,
causing hyponatremia. It also dilutes the serum, leading to decreased serum osmolality.
Because the aquaporin channels are actively pulling water out of the tubule, the remaining
urine becomes highly concentrated, resulting in an inappropriately high urine osmolality
(greater than 100 mOsm/kg) despite the serum being dilute.
, Question 2
A patient presents with central diabetes insipidus following a traumatic brain injury.
Contrast the pathophysiologic mechanism of central diabetes insipidus with nephrogenic
diabetes insipidus. Why does administering Desmopressin (a synthetic ADH analog) correct
the polyuria in central diabetes insipidus but has no effect on nephrogenic diabetes
insipidus?
Answer:
Central diabetes insipidus is caused by a deficiency in ADH production or release. This
usually occurs due to damage to the hypothalamic neurons (supraoptic and paraventricular
nuclei) or the posterior pituitary stalk from trauma, tumors, or surgery. Without ADH, the
collecting ducts remain impermeable to water. Large volumes of dilute fluid are excreted,
leading to polyuria and hypernatremia. Because the end organ (the kidney) is intact,
administering Desmopressin replaces the missing hormone, inserts the aquaporin
channels, and stops the polyuria. Nephrogenic diabetes insipidus is caused by an end-organ
resistance to ADH. The posterior pituitary is secreting normal or high levels of ADH, but the
kidneys cannot respond. This is typically due to genetic mutations affecting the V2
receptors or acquired defects (like lithium toxicity) that destroy the aquaporin channels.
Because the defect is at the receptor or channel level, adding more hormone
(Desmopressin) will have zero therapeutic effect.
Question 3
A 42-year-old female presents with galactorrhea, amenorrhea, and visual field defects. MRI
reveals a 1.8 cm pituitary macroadenoma. What specific anatomical structure is being
compressed to cause the visual field defect, and what is the classic visual field loss
observed? Furthermore, explain the pathophysiologic mechanism by which this specific
tumor causes amenorrhea.
Answer:
The pituitary gland sits in the sella turcica, directly inferior to the optic chiasm. As a
pituitary macroadenoma grows upward, it compresses the optic chiasm. The optic chiasm
contains the crossing fibers of the nasal retinas, which carry visual information from the
temporal (lateral) visual fields. Compression of these crossing fibers causes a bilateral loss
