Inappropriate ADH secretion is characterised by the following except –
**Core Concept**
Inappropriate antidiuretic hormone (ADH) secretion, also known as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), is a clinical disorder characterized by the excessive release of ADH, leading to water retention, hyponatremia, and dilutional hyponatremia. This condition is often caused by ectopic ADH production in various tumors, central nervous system disorders, or other pathologies.
**Why the Correct Answer is Right**
SIADH is characterized by the excessive release of ADH, which stimulates the kidneys to reabsorb water, leading to water retention and subsequent hyponatremia. The pathophysiology involves the inappropriate activation of the vasopressin 2 (V2) receptors in the collecting ducts of the kidneys, resulting in increased water reabsorption. This can be caused by various factors, including ectopic ADH production in tumors, such as small cell carcinoma of the lung, or central nervous system disorders, such as cerebral hemorrhage or infection.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because SIADH is characterized by excessive ADH release, not deficiency. ADH deficiency would lead to diabetes insipidus, characterized by polyuria and polydipsia.
* **Option B:** This option is incorrect because SIADH often presents with hyponatremia, not hypernatremia. Hyponatremia occurs due to the excessive water retention and dilutional effect on sodium levels.
* **Option C:** This option is incorrect because SIADH is often associated with a low serum osmolality, not high serum osmolality. The excessive water retention leads to a decrease in serum osmolality.
**Clinical Pearl / High-Yield Fact**
A key clinical feature of SIADH is the presence of hyponatremia in the absence of heart failure, liver disease, or nephrotic syndrome. This clinical scenario should prompt further investigation for SIADH, including measurement of ADH levels and evaluation for underlying causes.
**Correct Answer: A. ADH deficiency is the hallmark of diabetes insipidus, not SIADH.**