Radioimmunoassay of thyrotrophin releasing hormone in human serum and its clinical application.

A radioimmunoassay for thyrotrophin releasing hormone (TRH) in human serum is described. Inactivation of TRH immunoreactivity by serum is prevented by a mixture of 8-hydroxyquinoline sulphate and Tween 20. This persisted for a long period and at the concentration used in this study did not affect the TRH value. The TRH was extracted from serum with methanol. The results of the recovery experiment, dilution curve of high TRH serum, intra-assay reproducibility and inter-assay variation were quite satisfactory. The lowest detectable amount in this system was 5.0 pg/ml. Immunoreactive TRH levels in the serum were less than 60 pg/ml in normal subjects, were below the limit of detectability in hyperthyroid patients, were 40 to 400 pg/ml in primary hypothyroid patients, were 100 to 600 pg/ml in pituitary hypothyroid patients and below the limit of detectability in hypothalamic hypothyroid patients. In primary hypothyroidism, immunoreactive TRH concentrations in the serum fell to in the normal range during thyroxine substitution. In most cases of hyperthyroidism, immunoreactive TRH concentrations in the serum increased to 5.0 pg/ml or more during antithyroid drug treatment, but in a few cases the level remained below the limit of detectability even though thyroid hormone levels were in the normal range. The above data suggest that this assay system would be a useful tool to study the role of TRH in the hypothalamo-pituitary-thyroid axis.