Radioimmunoassay of Thyrotropin Releasing Hormone in Plasma and Urine

A sensitive and specific radioimmunoassay has been developed capable of measuring thyrotropin releasing hormone (TRH) in extracted human plasma and urine. All of three TRH analogues tested had little cross-reactivity to antibody. Luteinizing hormone releasing hormone, lysine vasopressin, rat growth hormone and bovine albumin were without effect, but rat hypothalamic extract produced a displacement curve which was parallel to that obtained with the synthetic TRH. Sensitivity of the radioimmunoassay was 4 pg per tube with intraassay coefficient of variation of 6.2-9.7%. Synthetic TRH could be quantitatively extracted by methanol when added to human plasma in concentration of 25, 50 and 100pg/ml. TRH immunoreactivity was rapidly reduced in plasma at 20°C than at 0°C, but addition of peptidase inhibitors, FOY-007 and BAL, prevented the inactivation of TRH for 3hr at 0°. The TRH in urine was more stable at 0° than 20°, and recovered 75±4.6% at 24 hr after being added. The plasma levels of TRH were 19pg/ml or less in normal adults and no sex difference was observed. The rate of disappearance of TRH administered i.v. from the blood could be represented as half-times of 4-12 min. Between 5.3-12.3% of the injected dose was excreted into urine within 1 hr as an immunoreactive TRH These results indicate the usefulness of TRH radioimmunoassay for clinical investigation.