Which sequence describes negative feedback in the hypothalamic-pituitary-thyroid axis?

• A. Elevated thyroid hormone levels inhibit TRH and TSH secretion
• B. Low thyroid hormone levels stimulate hypothalamus to release TRH and subsequently TSH, raising thyroid hormone production
• C. TRH directly inhibits TSH production
• D. TSH stimulates hypothalamus to produce more thyroid hormone

Answer: (A)

Negative feedback in this axis is when rising thyroid hormones signal the brain to dial back the signals that drive their production. The hypothalamus releases TRH, which tells the pituitary to secrete TSH, and TSH then prompts the thyroid to produce T3 and T4. When T3 and T4 levels get high, they suppress TRH and TSH release, reducing further thyroid hormone production and keeping levels balanced. The sequence where elevated thyroid hormone levels inhibit TRH and TSH secretion best describes this feedback inhibition. The other statements describe the initial drive to increase production when thyroid hormone is low or mix up which gland controls which step, so they don’t capture the feedback stopping point.