Which condition would most likely lead to dehydration due to polyuria?

• A. Diabetes insipidus
• B. SIADH
• C. Addison's disease
• D. Hyperthyroidism

Answer: (A)

Polyuria that leads to dehydration happens when the kidneys lose water because ADH action is deficient or ineffective. In diabetes insipidus, there is not enough vasopressin (central DI) or the kidneys don’t respond to it (nephrogenic DI). Without the antidiuretic hormone signal, the collecting ducts don’t reabsorb water, so urine becomes very dilute and can be produced in large volumes. If fluid intake doesn’t keep up with this loss, dehydration and hypernatremia can develop.

SIADH, by contrast, causes water retention and hyponatremia, with concentrated urine, not dehydration from polyuria. Addison’s disease involves salt wasting and volume depletion from aldosterone deficiency, but polyuria isn’t the defining feature. Hyperthyroidism can cause some increased urine output, but dehydration due to polyuria is not as characteristic as in diabetes insipidus.

So the condition most likely to cause dehydration from polyuria is diabetes insipidus.