Which feature best distinguishes a TSH-secreting pituitary adenoma (TSHoma) from thyroid hormone resistance syndrome as a cause of elevated TSH with elevated free T4?

• A Thyroid hormone resistance causes a more elevated free T4 than TSHoma
• B TSHoma responds completely to levothyroxine suppression
• C Thyroid hormone resistance is always associated with goitre
• D TSHoma characteristically shows elevated alpha-subunit of TSH and a mass on pituitary MRI ✓

Explanation

TSHoma produces excess intact TSH along with disproportionately elevated free alpha-subunit; an alpha-subunit to TSH molar ratio >1 is characteristic. Pituitary MRI confirms the adenoma. In thyroid hormone resistance (typically due to THRB mutations), there is no pituitary mass and the alpha-subunit is not elevated; the elevated TSH is a physiological response to receptor insensitivity. Levothyroxine suppression worsens TSHoma by reducing negative feedback but does not cure it.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.