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

## 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**