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: Unknown
[{'content': 'In pregnancy, which is the best parameter to be measured in thyroid function test? \nSelect and explain. \nA. TSH, B. Free t3, C. T4, D. Total t3', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref. The Short Textbook of Pediatrics. Pg 72\n\xa0\nFetal thyroid starts functioning after 12 weeks. Till then the fetus is entirely dependent upon the maternal supply of T4 through the placenta, for all neurologic developments .\n\nTRH crosses the placenta but TSH crosses it very minimally.\nMaternal total T3 and T4 are increased by 18 weeks but free T4\xa0 and T3 levels are unchanged .Maternal TSH levels remains normal.\nSecretion of T4 and T3 is 20: 1.\n\nHowever women with high risk factors should have their serum TSH level checked in their first antenatal visit', 'role': 'assistant'}]