hypocalciuria

Hypocalciuria is a condition characterized by abnormally low calcium excretion in the urine. In adults, a practical threshold is often 24-hour urinary calcium excretion below about 100 mg per day, though exact cutoffs vary by laboratory. The calcium-to-creatinine ratio in a spot urine sample or the fractional excretion of calcium (FeCa) can be used to assess urinary calcium, with FeCa values commonly reported and sometimes considered low when below 1%.

Causes and associations

The most well-recognized cause of hypocalciuria is familial hypocalciuric hypercalcemia (FHH), typically due to loss-of-function mutations in the calcium-sensing receptor (CASR) gene. In FHH, serum calcium is often mildly elevated or normal, while urinary calcium excretion is inappropriately low. Other contributors include the use of thiazide diuretics, which promote renal reabsorption of calcium, and, less commonly, reduced dietary calcium intake or conditions that limit calcium absorption. The exact interpretation of urinary calcium can be influenced by hydration status, renal function, and concurrent medications.

Clinical significance and evaluation

Hypocalciuria is most clinically relevant when evaluating hypercalcemia. A low urinary calcium excretion in the setting of hypercalcemia suggests FHH rather than primary hyperparathyroidism, which typically features higher urinary calcium. Distinction between these conditions is important because FHH generally does not respond to parathyroidectomy. Evaluation usually includes measurement of 24-hour urinary calcium, serum calcium and parathyroid hormone levels, review of medications, and, if indicated, genetic testing for CASR or related genes.

Management

Management depends on the underlying cause. FHH often requires no specific treatment beyond monitoring, while hypercalcemia from other etiologies is addressed by appropriate interventions. Thiazide-induced hypocalciuria may prompt reassessment of medication use.