text: TRH-stimulation testing however
  continues to be useful for the differential diagnosis of secondary (pituitary disorder) and tertiary (hypothalamic disorder) hypothyroidism. Patients with these conditions
  appear to have physiologically inactive TSH in their circulation that is recognized by
  TSH assays to a degree such that they may yield misleading, "euthyroid" TSH
  results. Use and Interpretation:
  • Helpful in diagnosis in patients with confusing TFTs. In primary hyperthyroidism
  TSH are low and TRH administration induces little or no change in TSH levels
  • In hypothyroidism due to end organ failure, administration of TRH produces a
  prompt increase in TSH
  • In hypothyroidism due to pituitary disease (secondary hypothyroidism) administration of TRH does not produce
  an increase in TSH
  • In hypothyroidism due to hypothalamic disease (tertiary hypothyroidism), administration of TRH produces a
  delayed (60–120 minutes, rather than 15–30 minutes) increase in TSH