CONTEXT: Treatment of primary hyperparathyroidism (PHPT) and secondary hyperparathyroidism due to idiopathic hypercalciuria (SHPT-IH) is markedly different. Robust diagnostic tools to differentiate between both entities are however lacking. OBJECTIVE: Evaluate the thiazide challenge test (TCT) in clinical practice, its aid in clinical decision making, and evaluate the accuracy (sensitivity, specificity) and potentially useful parameters of the TCT. METHODS: Monocentric observational retrospective cohort study from January 2017 to November 2023 in an outpatient Endocrinology department, Ghent University Hospital (Belgium). Twenty-five adult patients with hypercalciuria, elevated parathyroid hormone (PTH), and high-normal or elevated serum calcium underwent a TCT. Outcome measures were serum, urinary biochemical parameters before and after testing, clinical and imaging outcomes, treatment, and follow-up. RESULTS: Patients with a TCT-based working diagnosis of PHPT show greater increases in albumin-adjusted calcium and total serum calcium concentration than patients with SHPT-IH (+0.11 ± 0.10 vs +0.0071 ± 0.10 mmol/L
P = .025 and +0.14 ± 0.12 vs +0.012 ± 0.15 mmol/L
P = .024, respectively). The TCT-based working diagnosis of PHPT has a sensitivity of 81.8%, a specificity of 77.8%, and a likelihood ratio of 3.68 of estimating a correct final diagnosis. Urinary calcium excretion, PTH, calcium-phosphorous ratio, PTH inhibition rate, and the parathyroid function index do not differ significantly in patients with PHPT compared with those with SHPT-IH. CONCLUSION: The TCT aids in discriminating patients with PHPT from those with SHPT-IH based on a rise in serum calcium. Other parameters are not different between both groups. Larger prospective trials are necessary to further define the diagnostic potential of the TCT, its most appropriate biochemical outcome variables, and decision cut-offs.