This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'The most common cause of hypothyroidism in adults is: \nSelect and explain. \nA. Trauma, B. Radioactive iodine ingestion, C. Primary hypothyroidism, D. Parathyroid surgery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Primary hypothyroidism is the most common cause of hypothyroidism in adults.', 'role': 'assistant'}]