HPT Axis Physiology
A medical student is reviewing hypothalamic-pituitary-thyroid (HPT) axis regulation. Circulating T3 and T4 levels rise significantly. Which of the following sequences best describes the resulting hormonal response?
Correct Answer — B
The HPT axis operates by classical negative feedback. Rising T3/T4 inhibit both hypothalamic TRH secretion and pituitary TSH secretion simultaneously. With less TRH and TSH drive, the thyroid reduces T4 output — restoring equilibrium. This is a textbook closed-loop feedback: the signal (high T3/T4) directly suppresses the regulators above it.
Key concept
Negative feedback in the HPT axis: high T3/T4 → decreased TRH → decreased TSH → decreased thyroid hormone output.