If TRH is administered and TSH levels remain low, this suggests which disorder?

• A. Primary hypothyroidism
• B. Secondary hypothyroidism
• C. Tertiary hypothyroidism
• D. Iodine deficiency

Answer: (B)

The TRH stimulation test helps localize where the problem lies along the hypothalamic-pituitary-thyroid axis. TRH from the hypothalamus prompts the pituitary to release TSH, which then signals the thyroid to produce thyroid hormones. If you administer TRH and TSH stays low, the pituitary isn’t responding to TRH, pointing to a pituitary defect—secondary hypothyroidism. If the pituitary could respond normally, TSH would rise after TRH. In hypothalamic (tertiary) problems, giving TRH would still cause the pituitary to release TSH, so TSH would rise. In primary thyroid disease or iodine deficiency, the issue is at the thyroid, where TSH would typically respond to TRH from the pituitary, even though thyroid hormone output is low. So a lack of TSH rise after TRH specifically indicates pituitary dysfunction, i.e., secondary hypothyroidism.