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 principal impact indicator in Iodine Deficiency disorders 
Select and explain. 
A. Urinary Iodine Levels, B. Goitre assessment, C. Neonatal TSH levels, D. Salt Iodine Content
A: Urinary Iodine Levels
Answer: A: Urinary Iodine Levels 
Explanation: Urinary Iodine Levels - principal impact indicator.
Neonatal TSH levels - screening programme for hypothyroidism.