What diagnostic workup should I perform for a 57-year-old female new patient presenting with polyuria, polydipsia, weakness, fatigue, and dizziness that began after a streptococcal infection, with thyroid disease and type 2 diabetes already excluded?

Workup for Diabetes Insipidus Following Streptococcal Infection

You should immediately measure serum sodium, serum osmolality, and urine osmolality as your initial biochemical workup, followed by a water deprivation test with desmopressin challenge to differentiate between central and nephrogenic diabetes insipidus. 1

Initial Laboratory Assessment

Your patient's presentation—polyuria, polydipsia, weakness, fatigue, and dizziness following an infection in a previously active individual—strongly suggests diabetes insipidus (DI). The temporal relationship with streptococcal infection is particularly noteworthy, as viral and bacterial infections can trigger central DI 2.

Start with these specific tests:

• Serum sodium concentration
• Serum osmolality
• Simultaneous urine osmolality
• 24-hour urine volume collection (to quantify polyuria; expect >3L/day if DI is present) 3

Key diagnostic thresholds to look for:

• Urine osmolality <200 mOsm/kg H₂O strongly suggests DI 1
• Elevated or high-normal serum osmolality (>295 mOsm/kg) with inappropriately dilute urine confirms the diagnosis 4
• Hypernatremia may be present if fluid intake hasn't kept pace with losses

Distinguishing Central from Nephrogenic DI

Once you've confirmed hypotonic polyuria (urine osmolality <150-200 mOsm/kg), proceed with a supervised water deprivation test with desmopressin challenge 5, 2. This remains the gold standard for differentiation:

Water deprivation test interpretation:

• Central DI: Urine osmolality increases >50% after desmopressin administration
• Nephrogenic DI: Minimal or no response (<10% increase) to desmopressin
• Primary polydipsia: Urine concentrates appropriately during water deprivation before desmopressin is needed

Important caveat: If copeptin measurement is available at your institution, it can enhance diagnostic accuracy and potentially reduce the need for prolonged water deprivation testing 2. However, this is not routinely available in most centers.

Additional Workup Based on Initial Findings

If Central DI is Confirmed:

Obtain MRI of the pituitary and hypothalamus to evaluate for:

• Pituitary stalk lesions
• Hypothalamic masses
• Loss of posterior pituitary bright spot on T1-weighted images
• Infiltrative processes (sarcoidosis, histiocytosis)

The post-infectious timing suggests possible autoimmune or inflammatory etiology 2.

If Nephrogenic DI is Suspected:

Check for secondary causes:

• Serum calcium (hypercalcemia causes nephrogenic DI)
• Serum potassium (hypokalemia impairs concentrating ability)
• Medication review (lithium is the classic culprit, but also amphotericin B, demeclocycline)
• Consider genetic testing of AVPR2 and AQP2 genes in females if no secondary cause identified 1

Critical Pitfalls to Avoid

1. Don't assume diabetes mellitus is controlled just because it was "ruled out"—recheck glucose and HbA1c, as osmotic diuresis from hyperglycemia can mimic DI 4. Even well-controlled diabetics can develop concurrent DI.

2. Monitor closely during water deprivation testing—stop if serum sodium exceeds 145 mEq/L or serum osmolality exceeds 300 mOsm/kg to prevent dangerous hypernatremia 5.

3. Don't overlook partial DI—patients may have incomplete deficiency with urine osmolality between 200-600 mOsm/kg, making diagnosis more challenging 5.

4. Consider rare mimics—pheochromocytoma can present with polyuria-polydipsia syndrome 5, though less likely given your patient's presentation.

Algorithmic Approach Summary

1. Confirm polyuria: 24-hour urine collection (>3L/day)
2. Initial labs: Simultaneous serum and urine osmolality, serum sodium
3. If urine osmolality <200 mOsm/kg with elevated/normal serum osmolality: Proceed to water deprivation test
4. Based on desmopressin response:
   • Good response (>50% increase) → Central DI → MRI brain
   • Poor response (<10% increase) → Nephrogenic DI → Check calcium, potassium, medications, consider genetics
5. Initiate treatment based on etiology while completing workup

The post-infectious onset in your patient makes central DI most likely, potentially from autoimmune hypophysitis triggered by the streptococcal infection 2. This would be responsive to desmopressin therapy and could dramatically improve her quality of life and ability to return to marathon training.