A 54-year-old man with a lung tumor has high circulating levels of antidiuretic hormone (ADH), a serum osmolarity of $260 \mathrm{mOsm} / \mathrm{L}$, and a negative free-water clearance $\left(\mathrm{C}_{\mathrm{H}_{2} \mathrm{O}}\right)$. Which diagnosis is correct?

Question

Quizplus · Accepted Answer

[Chapter 5, VII D 4]. A negative value for free-water clearance $\left(\mathrm{C}_{\mathrm{H}_{2} \mathrm{O}}\right)$ means that "free water" (generated in the diluting segments of the thick ascending limb and early distal tubule) is reabsorbed by the collecting ducts. A negative $\mathrm{C}_{\mathrm{H}_{2} \mathrm{O}}$ is consistent with high circulating levels of antidiuretic hormone (ADH). Because ADH levels are high at a time when the serum is very dilute, $\mathrm{ADH}$ has been secreted "inappropriately" by the lung tumor.

A 54-Year-Old Man with a Lung Tumor Has High Circulating 260mOsm/L260 \mathrm{mOsm} / \mathrm{L}260mOsm/L

A 54-Year-Old Man with a Lung Tumor Has High Circulating $260 \mathrm{mOsm} / \mathrm{L}$