Thyrotropin-releasing hormone cannot be detected in plasma from normal subjects.

Different laboratories report widely varying average levels of TRH in plasma from normal man. However, arguments can be raised against the methods used to characterize the substances measured in all previous studies. Using a sensitive RIA and methanol extraction, immunoreactive TRH (iTRH) in serum from normal subjects averaged 49 +/- 26 pg/ml (mean +/- SD; n = 6). When SP-Sephadex purification was introduced, iTRH decreased to 1.8 +/- 0.7 pg/ml. These low quantities did not further decrease when incubated with TRH-degrading enzymes, demonstrating that the remaining iTRH was not identical to synthetic TRH. Investigations of iTRH in pools of normal plasma indicate that the level of true TRH in normal human plasma is below 0.4 pg/ml.