You are seeing a 34-year-old female in A&E who has presented with headache, nausea, anxiety and weight loss. This patient informs you that these symptoms came on suddenly a couple of months ago and have been on and off but getting worse over the last few weeks. On examination you note that she is sweating profusely, has a tremor and has a heart rate of 129 and a blood pressure of 167/95. A blood test confirms that this patient has raised calcium levels, raised haemoglobin levels and a raised blood glucose level. What is the most likely diagnosis?