Patients referred to a specialist heat illness clinic (n = 2 with prior hyponatremia; n = 16 lacking documentary evidence) and controls (n = 16) underwent laboratory Heat Tolerance Assessment (HTA: 60–90 min walking, 60% relative intensity, 34°C heat), synthetic adrenocorticotrophic hormone stimulation (heat illness only) and CYP21A2 genotyping (hyponatremic heat illness only). The gene discussed is CYP21A2; the disease is Hyponatremia.