Diabetes Insipidus: The Daily PANCE Blueprint
A patient on long-term lithium for bipolar disorder develops polyuria, polydipsia, & a high serum Na+. What is the mechanism of this side effect?
A. Decreased ADH production
B. ADH resistance at the collecting duct
C. Increased aldosterone secretion
D. Increased glucose reabsorption
E. Aldosterone antagonism
Answer and topic summary
The answer is B. ADH resistance at the collecting duct
A major long-term side effect of lithium is nephrogenic diabetes insipidus (NDI). Lithium accumulates in the collecting duct cells of the kidney, where it interferes with the cyclic AMP second messenger system. This makes the tubules resistant to the effects of antidiuretic hormone (ADH). Without a proper response to ADH, the kidneys cannot effectively reabsorb water, leading to the excretion of large volumes of dilute urine (polyuria) and compensatory intense thirst (polydipsia).
Incorrect Answer Explanations:
- A. Decreased ADH production is the mechanism of central diabetes insipidus, which is a problem with the hypothalamus or posterior pituitary, not a side effect of lithium on the kidney.
- C. Increased aldosterone secretion (hyperaldosteronism) would cause sodium retention and potassium excretion, leading to hypertension and hypokalemia.
- D. Increased glucose reabsorption is not a known mechanism of lithium and is unrelated to diabetes insipidus.
- E. Aldosterone antagonism is the mechanism of action for potassium-sparing diuretics like spironolactone. Lithium does not work this way.
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Smarty PANCE Content Blueprint Review:
Covered under ⇒ PANCE Blueprint Endocrinology ⇒ ⇒
Also covered as part of the Internal Medicine EOR and Emergency Medicine PAEA EOR topic list
