Introduction: Aggression in various forms may be one of the components of emotional/behavioural problems seen in a Child and Adolescent Mental Health Service. Aggressive symptoms may result from a psychiatric condition or a physical illness such as Primary Hyperparathyroidism. Case Presentation: The authors present a case of worsening psychiatric symptoms in a patient who was serendipitously diagnosed with Primary Hyperparathyroidism. His aggressive outbursts, suicidal and homicidal thoughts remitted following parathyroidectomy. Conclusion: Primary Hyperparathyroidism should be considered in the differential diagnosis in young people with worsening neuropsychiatric symptoms which are unresponsive to standard psychiatric treatment. Child Psychiatrists should always take holistic approach when managing patients and should be familiar with medical conditions that present with psychiatric symptoms.