A 40-year-old woman with a TSH-secreting pituitary adenoma has elevated free T4 (18 pg/mL, normal 0.7–1.9) and elevated TSH (12 mIU/L, normal 0.4–4). Which mechanism explains the elevated TSH despite thyroid hormone excess?

Question

Accepted Answer

D. Autonomous TSH secretion by the adenoma, resistant to negative feedback. ## TSH-Secreting Pituitary Adenoma (Thyrotropinoma)

**Pathophysiology:**
- Rare pituitary adenoma (<3% of adenomas) that autonomously secretes TSH
- TSH secretion is **independent of normal negative feedback** from thyroid hormones
- Results in the paradoxical combination of:
  - Elevated TSH (from tumor)
  - Elevated free T4 (from TSH stimulation of thyroid)
  - Elevated free T3

**Key Point:** This is the **only condition** where both TSH and free T4 are elevated simultaneously. In all other causes of hyperthyroidism, TSH is suppressed.

**Diagnostic Clues:**
- Elevated TSH + elevated free T4 (paradoxical)
- MRI shows pituitary adenoma
- TSH levels do not suppress with exogenous thyroid hormone
- Elevated alpha-subunit (common in TSH-secreting adenomas)

**Clinical Pearl:** Thyrotropinomas are often diagnosed late because the elevated TSH can be mistaken for primary hypothyroidism, leading to inappropriate levothyroxine therapy (which worsens hyperthyroidism).

**Mnemonic:** **TSH + T4 both ↑ = Think Thyrotropinoma**

Answer

A. Increased TRH secretion from the hypothalamus

Answer

B. Primary hypothyroidism with secondary pituitary hypertrophy

Answer

C. Thyroid hormone resistance at the pituitary level

A 40-year-old woman with a TSH-secreting pituitary adenoma has elevated free T4 (18 pg/mL, normal 0.7–1.9) and elevated TSH (12 mIU/L, normal 0.4–4). Which mechanism explains the elevated TSH despite thyroid hormone excess?

Explanation

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