UNLABELLED: Loss-of-function mutations in ATP-sensitive potassium (KATP) channels cause hyperexcitability and insulin hypersecretion, resulting in congenital hyperinsulinism (CHI). Paradoxically, despite the initial insulin hypersecretion, many CHI cases, as well as KATP knockout (KO) animals, eventually "crossover" to undersecretion and even diabetes. Here, we confirm that Sur1 KO islets exhibit higher intracellular concentration of calcium ion ([Ca2+]i) at all concentrations of glucose but show decreased glucose-stimulated insulin secretion. However, when [Ca2+]i is artificially elevated by increasing extracellular [Ca2+], insulin secretion from Sur1 KO islets increases to the same levels as in wild-type (WT) islets. This indicates that a right-shift in [Ca2+]i dependence of insulin secretion, rather than loss of insulin content or intrinsic secretability, is the primary cause for the crossover. Chronic pharmacological inhibition of KATP channel activity by slow release of glibenclamide in pellet-implanted mice causes a very similar crossover to glucose intolerance and impaired insulin secretion seen in Sur1 KO animals. Whole-islet and single-cell transcriptomic analysis reveal markedly reduced Trpm5 in both conditions. Glibenclamide pellet-implanted Trpm5 KO mice also exhibited significant glucose intolerance. However, this was not as severe as in WT animals, which suggests decreased expression of Trpm5 may play a small role in the disruption of insulin secretion with KATP loss. ARTICLE HIGHLIGHTS: Congenital hyperinsulinism caused by loss of ATP-sensitive potassium (KATP) channels crosses over to unexplained undersecretion. Why does loss of β-cell KATP channel activity result in undersecretion of insulin and glucose tolerance, despite elevated intracellular concentration of calcium ion ([Ca2+]i) levels? Superelevation of [Ca2+]i in supraphysiological extracellular [Ca2+] boosted secretion from Sur1 knockout (KO) islets to the same levels as WT, indicating a right-shift in [Ca2+]i dependence of secretion. Transcriptomic analysis revealed markedly reduced β-cell Trpm5 in the absence of KATP. KATP inhibition in Trpm5 KO mice still caused significant glucose intolerance, but slightly less severe than in WT animals. Right-shifted [Ca2+]i dependence of secretion explains crossover. Downregulation of Trpm5 may be involved.