Calcitonin, released by C-cells of the thyroid gland in response to increased serum ionised calcium concentration (iCa), protects against hypercalcaemia by inhibition of osteoclasts, but does not have an important physiologic role in all species. In cats, experimentally-induced hypercalcaemia elicits a calcitonin response in some but not all animals, possibly causing non-responders to be more susceptible to hypercalcaemia. Ionised hypercalcaemia is increasingly recognised in cats, some of which have chronic kidney disease (CKD). The aim of this study was to explore the calcitonin response to ionised hypercalcaemia in cats with CKD. Twenty-eight cats diagnosed with CKD (including 13 cats developing hypercalcaemia within 8 months) were selected for this retrospective observational cohort study. iCa >1.40 mmol/L was classified as hypercalcaemia. Calcitonin was measured by