This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
[{'content': 'Single best test for diagnosis of hypothyroidism is estimation of \nSelect and explain. \nA. T3, B. T4, C. TSH, D. RAIU', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. c (TSH). (Ref. Harrison, Principles of Medicine, 15th ed. 2066)# A normal TSH level excludes primary (but not secondary) hypothyroidsm.# If TSH is elevated, a free T4 level is needed to confirm presen ce of clinical hypothyroidism, but free T4 is inferior to TSH when used as screening test, as it will not detect subclinical or mild hypothyroidsm.# Circulating free T3 levels are normal in about 25% patients, reflecting adaptive response to hypothyroidism. T3 levels are therefore not indicated.# For congenital hypothyroidism even TSH/T4 levels in heel-prick blood specimens are used for neonatal screening.# Note: Iodine deficiency remains the most common cause of hypothyroidsm.', 'role': 'assistant'}]