MCAT Biology Question 66: Answer and Explanation

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Question: 66

6. A patient has a very high TSH level. Which of the following would NOT cause a high TSH level?

  • A. Autoimmune destruction of thyroid cells that produce T3 and T4.
  • B. A tumor in the hypothalamus that secretes high levels of TRH.
  • C. High levels of T4 from thyroid replacement medications.
  • D. Cancerous growth of parafollicular cells in the thyroid, destroying other cell types in the organ.

Correct Answer: C

Explanation:

The hypothalamic–pituitary–thyroid axis includes the secretion of thyroid-releasing hormone (TRH) from the hypothalamus stimulating the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary, which stimulates the secretion of triiodothyronine (T3) and thyroxine (T4) from the thyroid. Overproduction of TRH would promote overproduction of TSH, eliminating choice (B). Destruction of the follicular cells that produce T3 and T4 would remove negative feedback, allowing TSH levels to rise, eliminating choices (A) and (D). High levels of T4 would cause too much negative feedback and lower TSH levels, making choice (C) the correct answer.

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