Validation of a heterologous radioimmunoassay for the determination of rat thyrotrophic hormone.

A heterologous RIA system is described for the determination of TSH in rat plasma. The antibody was obtained by us in guinea-pigs and directed against bovine-TSH (B-TSH). It was chosen from several antisera as the one showing the greatest potency in vivo against rat TSH, using the McKenzie bioassay. Two different antigens were tried for radioiodination: B-TSH (B-TSH) and a purified mouse tumour TSH (M-TSH). The choice of labeled antigen proved to be critical. It was not possible to develop a reliable RIA with B-TSH. Using M-TSH, however, a RIA was developed which is sensitive enough to detect differences between normal and lower than normal plasma TSH levels. The reproducibility, sensitivity and specificity of the RIA are described, as well as several procedures which shorten the time spent on a given assay, and at the same time decrease inter and intra-assay variations. Some of the results obtained with experimental plasmas were compared to the in vivo potency of the samples in the McKenzie bioassay. The results obtained with the present RIA have also been validated physiologically by carrying out the determinations on plasmas obtained from rats submitted to situations known to decrease (hypophysectomy, treatment with thyroid hormones, ether anaesthesia) or increase (injection of TRH, thyroidectomy, treatment with goitrogens) circulating TSH levels. The circulating TSH levels of normal rats found with the present RIA compare well with values obtained with the homologous immunoreactants available from NIAMDD at NIH (U.S.A.).