Endocrine System MCAT Practice Question
A 55-year-old woman presents with a 3-month history of fatigue, weight gain, cold intolerance, and constipation. Vital signs are unremarkable. Laboratory studies reveal TSH 8.5 mIU/L (normal 0.5-5.0), free T4 0.9 ng/dL (normal 0.8-1.8), and positive thyroid peroxidase (TPO) antibodies at 287 IU/mL (normal <35). A diagnosis of Hashimoto thyroiditis is made and levothyroxine 50 mcg daily is initiated. After 6 weeks, TSH normalizes to 1.2 mIU/L and her symptoms improve. On follow-up 6 months later, when she asks about discontinuing the medication, you explain that long-term thyroid hormone replacement will likely be necessary. Which of the following best explains the need for continued levothyroxine therapy in this patient?
Answer choices
- ALevothyroxine suppresses TSH, which prevents the remaining thyroid tissue from regenerating and producing adequate endogenous hormone
- BPositive TPO antibodies indicate ongoing autoimmune-mediated thyroid destruction, resulting in progressive loss of thyroid function that requires permanent hormone replacementCorrect answer
- CLevothyroxine increases hepatic metabolism of cortisol, necessitating concurrent steroid replacement to maintain adequate adrenal function
- DChronic levothyroxine administration induces thyroid peroxidase enzyme expression, making the gland increasingly dependent on exogenous hormone
- EThe patient has developed central hypothyroidism, in which the pituitary fails to produce TSH despite adequate thyroid hormone levels
- FLevothyroxine enhances intestinal iodine absorption during treatment, but discontinuation causes rapid depletion of iodine stores and recurrent hypothyroidism
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