A 44-year-old man has free T4 of 3.0 ng/dL, free T3 of 8.2 pg/mL, and a nonsuppressed TSH of 3.4 mIU/L. Pituitary MRI shows a 9 mm macroadenoma. Which additional finding best distinguishes a TSH-secreting pituitary adenoma from thyroid hormone resistance?
- A Failure of TSH to respond to TRH stimulation
- B Elevated molar ratio of alpha subunit to intact TSH ✓
- C Presence of a pituitary microadenoma on MRI
- D Family history of similar biochemical abnormalities
Correct answer: B. Elevated molar ratio of alpha subunit to intact TSH
Explanation
TSH-omas co-secrete excess free alpha subunit, giving an alpha subunit to TSH molar ratio above 1, whereas thyroid hormone resistance shows no alpha subunit excess. Failure of TSH response to TRH supports an adenoma but the test is unavailable in many countries and less specific. Thyroid hormone resistance is caused by THRB mutations, is often familial, and may show incidental pituitary anomalies, so imaging alone does not settle the diagnosis.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.