Thyroid Disorders USMLE Step 1 Practice Question
A 28-year-old woman presents with a 6-month history of amenorrhea, tremor, heat intolerance, and palpitations. On examination, her thyroid is diffusely enlarged and nontender. Vital signs show resting heart rate of 98 bpm. Laboratory studies reveal: TSH: 0.2 mIU/L (normal 0.4-4.0)
Free T4: 3.2 ng/dL (normal 0.8-1.8)
Free T3: 8.5 pg/mL (normal 2.3-4.2)
LH: 8 mIU/mL (normal follicular phase 1.9-12.5)
FSH: 6 mIU/mL (normal follicular phase 3.1-8.7)
Prolactin: 12 ng/mL (normal <25) Thyroid ultrasound demonstrates diffuse parenchymal enlargement with increased vascularity. Which of the following best explains the mechanism of amenorrhea in this patient?
Answer choices
- AThyroid peroxidase antibodies directly attack ovarian tissue causing autoimmune destruction
- BExcess thyroid hormone increases the metabolic clearance of estrogen, resulting in decreased circulating estrogen levels
- CThyroid hormone directly suppresses GnRH secretion from the hypothalamus, resulting in decreased LH and FSHCorrect answer
- DTSH receptor antibodies cross-react with FSH receptors on ovarian granulosa cells
- EExcess thyroid hormone increases hepatic synthesis of sex hormone-binding globulin, reducing free estrogen bioavailability
- FExcessive thyroid hormone stimulates central prolactin secretion, inhibiting pulsatile GnRH release
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