TRH test can play a supportive role in confirming the suspicion of mild CeH and differentiating tertiary (hypothalamic) from secondary (pituitary) hypothyroidism, as these conditions may present with exaggerated, delayed (peak after 60 min), or prolonged, versus blunted (< 4 mU/L) TSH responses to TRH 200 mcg i.v injection, respectively [50]. This evidence concerns the gene TRH and hypothyroidism.