Myxoedema coma is a life-threatening manifestation of severe, untreated hypothyroidism. It occurs when the body's compensatory mechanisms for coping with low thyroid hormone levels are overwhelmed, such as by infection, drugs, or other illness. Left untreated, it can affect multiple organ systems as the adaptations that help maintain homeostasis start to fail. Symptoms include deterioration of mental status, reduced brain function, hypothermia, slowed metabolism and reflexes, bradycardia, edema, and respiratory issues. Prompt diagnosis and treatment are needed to avoid permanent brain damage or death.