In primary hypothyroidism, which statement best describes the expected changes in pituitary and hypothalamic signaling?

• A. TSH and TRH both decrease.
• B. TSH and TRH both increase.
• C. TSH increases while TRH decreases.
• D. TSH decreases while TRH increases.

Answer: (B)

In primary hypothyroidism, the thyroid gland isn’t producing enough thyroid hormone (T4/T3). That fall in circulating thyroid hormones removes the normal negative feedback on both the hypothalamus and the pituitary. As a result, the hypothalamus increases TRH release, and the pituitary responds to low thyroid hormone by secreting more TSH. Since TRH stimulates TSH, both signals rise. Clinically, this shows up as high TSH with low T4, and TRH levels aren’t usually measured but would be elevated in this scenario. If the problem were upstream (hypothalamic or pituitary failure), the patterns would be different (low TSH with low or inappropriately normal thyroid hormones for secondary/tertiary causes).