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
The most common cause of hypothyroidism in adults is: 
Select and explain. 
A. Trauma, B. Radioactive iodine ingestion, C. Primary hypothyroidism, D. Parathyroid surgery
C: Primary hypothyroidism
Answer: C: Primary hypothyroidism 
Explanation: Primary hypothyroidism is the most common cause of hypothyroidism in adults.