This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Nutritional Deficiencies, Subject name: Social & Preventive Medicine
The most sensitive indicator of 'Environmental iodine deficiency' is:- 
Select and explain. 
A. Serum T4 levels, B. Serum T3 levels, C. Urine iodine excretion, D. Neonatal hypothyroidism
D: Neonatal hypothyroidism
Answer: D: Neonatal hypothyroidism 
Explanation: Neonatal hypothyroidism is the most sensitive indicator of environmental iodine deficiency. Other useful indicators for assessment of Iodine deficiency: Prevalence of goiter Prevalence of cretinism Urinary iodine excretion (Major impact indicator) Measurement of thyroid function by determination of serum T4, TSH.