CASR and parathyroid gland adenoma: This evidence of intratumoural heterogeneity in the composition and biochemical behaviour of parathyroid adenomas calls into question the assumption of CASR down‐regulation as an obligate mechanism in PHPT and highlights the need for a means to interrogate intrinsic calcium responsiveness of individual parathyroid tumour cells as a more direct functional readout of the underlying calcium sensing deficit.