<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 34-year-old female presents to her primary care physician complaining of fatigue. Over the last three months she has experienced decreased energy and gained 7 pounds. Review of systems is negative for symptoms of depression but is positive for constipation, myalgias, and cold intolerance. Physical exam is notable for delayed deep tendon reflex relaxation. Vital signs are as follows: T 37.1 C, HR 61, BP 13288, RR 16, and SpO2 100. Which of the following is the best initial screening test for this patient?? A: Level of anti-thyroid peroxidase TPO antibodies, B: Morning cortisol and plasma ACTH, C: Serum Free T4, D: Serum TSH, E: Hemoglobin and hematocrit,  <response>: This is a clinical vignette describing a patient with symptoms of hypothyroidism, including fatigue, weight gain, constipation, myalgias, cold intolerance, and delayed deep tendon reflex relaxation. The best initial screening test for suspected hypothyroidism is measurement of thyroid stimulating hormone TSH. Anti-TPO antibodies can help diagnose autoimmune thyroiditis but are not the first step. Morning cortisol and ACTH would be indicated for suspected adrenal insufficiency but are not relevant here. Free T4 is useful but TSH is more sensitive for picking up early hypothyroidism. Hemoglobin and hematocrit may help diagnose anemia but do not screen for hypothyroidism specifically.D: Serum TSH 