The greatest diagnostic value of TRH is in the differential diagnosis of thyrotoxicosis in borderline cases of mild thyroid overactivity. Since thyroid hormones inhibit TSH response to TRH, TRH effects are blunted or blocked in the presence of even mild degrees of hyperthyroidism. In current practice, the TRH test, which had been widely used in the evaluation of difficult diagnostic problems, has been largely supplanted by the ultra sensitive TSH immunoassays that allow measurement of basal or unstimulated TSH values in the full range of normal and below.
