Question

A patient with 3 months of polyuria has serum osmolality of 280, urine osmolality of 300, serum sodium 140, and no nocturnal polyuria, no H/o previous medications. What is the most likely diagnosis?

• A. Central polydipsia
• B. Nephrogenic polydipsia
• C. Primary polydipsia
• D. Bardet Biedl syndrome

Hint:

Differentiating Polyuria:
Diabetes Insipidus (Central/Nephrogenic): High serum sodium, High serum Osmolality, VERY prominent nocturia.
Primary Polydipsia: Low/Normal serum sodium, Low/Normal serum Osmolality, usually LACKS nocturia (because they don't drink while asleep).

Answer 1

The Correct Option is C

Concept:
A patient presents with a chronic history of significant polyuria (excessive urination). A basic metabolic panel and paired serum/urine osmolality tests yield completely normal to low-normal values, and there is a conspicuous absence of nocturia.
The clinical task is to interpret these specific fluid-status laboratory parameters to accurately differentiate between the three major causes of severe polyuria: Central Diabetes Insipidus, Nephrogenic Diabetes Insipidus, and Primary Polydipsia.
Explanation:
• Polyuria is broadly defined as a urine output exceeding 3 liters per day. The critical first step in diagnosis is determining whether the polyuria is driven by a primary defect in renal water conservation (Diabetes Insipidus) or by a primary excessive intake of water (Primary Polydipsia).

• In both Central and Nephrogenic Diabetes Insipidus (DI), the fundamental issue is a profound inability to concentrate urine due to either a lack of ADH (Central) or profound renal resistance to ADH (Nephrogenic).

• Because these DI patients are constantly, uncontrollably losing massive amounts of free water into their urine, their serum inherently becomes highly concentrated. Therefore, in classical untreated DI, the serum sodium is typically high-normal or frankly elevated (hypernatremia), and the serum osmolality is abnormally high (typically $> 295$ mOsm/kg).

• In sharp contrast, Primary Polydipsia (Option C), also known as psychogenic polydipsia, is fundamentally driven by a primary, often compulsive, excessive consumption of massive volumes of water.

• This massive water intake rapidly dilutes the blood, actively suppressing endogenous ADH release, which then correctly and physiologically results in the excretion of large volumes of dilute urine (polyuria).

• Because the driving force is excess water intake, these patients inherently present with a low-normal or frankly low serum sodium (this patient is at 140 mEq/L, which is normal, not high) and a normal to low serum osmolality (this patient is at 280 mOsm/kg).

• Furthermore, a critical, classic clinical clue provided is the explicit absence of nocturia (nocturnal polyuria). Patients with Primary Polydipsia do not typically drink water while they are asleep; therefore, they do not produce massive urine volumes at night. In stark contrast, patients with true Diabetes Insipidus suffer from a relentless, structural concentrating defect that persists relentlessly throughout the night, forcing them to wake up frequently to urinate.
Final Answer:
The specific combination of daytime polyuria, the absence of nocturia, and a low-normal serum osmolality strongly points to excessive fluid intake as the primary driver, definitively diagnosing Primary Polydipsia.