Endocrine System MCAT Practice Question
A 34-year-old woman presents with a 6-week history of palpitations, heat intolerance, and unintentional weight loss of 8 pounds despite increased appetite. She reports tremor and anxiety. Vital signs show heart rate of 112 bpm and blood pressure of 148/92 mmHg. On examination, she has a diffuse thyroid enlargement without nodules, mild proptosis, and pretibial skin thickening. Laboratory studies reveal: Free T4: 3.2 ng/dL (normal: 0.8–1.8)
Free T3: 8.5 pg/mL (normal: 2.3–4.2)
TSH: 0.01 mIU/L (normal: 0.5–5.0)
TPO antibodies: Elevated
TSH receptor antibodies: Elevated Which of the following best explains the suppressed TSH level in this patient?
Answer choices
- ATSH receptor antibodies bind to TSH receptors on thyroid follicular cells, directly stimulating thyroid hormone production and preventing TSH-mediated feedback
- BElevated circulating free T4 and T3 exert negative feedback inhibition on TRH secretion in the hypothalamus and TSH secretion in the anterior pituitaryCorrect answer
- CAutoimmune destruction of thyrotroph cells in the anterior pituitary reduces the pituitary's capacity to synthesize TSH
- DTSH receptor antibodies cross-react with TSH receptors on pituitary thyrotroph cells, blocking TSH secretion
- EIncreased sympathetic nervous system activity from excess thyroid hormones stimulates hypothalamic dopamine release, which inhibits TRH and TSH secretion
- FCirculating TSH is sequestered by excess thyroid hormones, rendering it biologically inactive and unmeasurable by standard assays
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