Which of the following CORRECTLY describes the feedback control of the hypothalamo-pituitary-thyroid axis that distinguishes tertiary from secondary hypothyroidism?

• A In secondary hypothyroidism, TSH is low or inappropriately normal, and TRH stimulation test shows blunted TSH rise
• B In tertiary hypothyroidism, TSH is elevated and TRH stimulation test shows exaggerated TSH rise
• C Both secondary and tertiary hypothyroidism have low TSH; differentiated by a delayed peak TSH response (at 60 minutes instead of 30 minutes) to TRH in tertiary hypothyroidism ✓
• D Tertiary hypothyroidism shows elevated TSH because TRH deficiency eliminates negative feedback inhibition

Explanation

Secondary hypothyroidism arises from pituitary failure (low TSH, blunted or absent TSH response to exogenous TRH). Tertiary hypothyroidism arises from hypothalamic TRH deficiency; the pituitary is intact but chronically understimulated, resulting in low but functional TSH. On TRH stimulation, the pituitary responds (unlike secondary), but the peak TSH response is delayed — occurring at 60 minutes rather than the normal 20–30 minutes — because unstimulated thyrotrophs take longer to mobilize stored TSH. Option A describes secondary, not the distinction. Option B erroneously states tertiary has elevated TSH. Option D is incorrect because TRH stimulates TSH release; its absence would lower TSH.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.