Diabetes Insipidus

Among following options all are the causes of hyponatremia (low Na) except one

A) Diabetes insipidus

B) Symptom of Inappropriate ADH secretion 

C) Hypothyroidism

D) Nephrotic syndrome

E) None of the above

Correct: A (Diabetes insipidus)

In diabetes insipidus lack of effects of antidiuretic hormone in body (due to central deficiency or peripheral receptor resistance) will lead to free water loss from body. This will lead to electrolyte abnormalities including

1) Hypernatremia

2) Hyperchloremia

To the contrary, retaining water due to excessive ADH (SIADH) will lead to hyponatremia. Increased free water in body will mediate loss of sodium via urine causing increased urinary sodium, increased urinary osmolality and at the same time decreased serum sodium and decreased serum osmolality.

In hypothyroidism, decreased thyroid levels cause decreased action of endogenous catecholamine on heart causing decrease in cardiac output. This decreased output is sensed by kidneys that causes increased ADH secretion (causing free water absorption). There is also increased sympathetic activity but lack of thyroid hormones renders most of sympathetic activity ineffective. Hence no sodium resorption occurs. Rather, free water absorption by ADH coupling with urinary loses of sodium leads to hyponatremia. In nephrotic syndrome, hypervolemic hyponatremia Occurs.