Validation of a radioimmunoassay for rat thyrotrophic hormone. II. Comparison with results obtained by bioassay.

As previously reported a heterologous radioimmuoassay (RIA) system was developed for the determination of rat thyrotrophic hormone (TSH), using guinea-pig antiserum against bovine TSH and a purified murine TSH for labeling. With this RIA system it is possible to detect TSH in plasma from normal rats, to follow its decrease during ether anaesthesia, and to differentiate between the TSH levels of normal rats and those of hyphophysectomized or thyroid hormone-treated rats; in appears to be free from interferences due to high circulating gonadotrophin levels. Several preparations containing high concentrations of rat TSH were assayed simultaneously by this RIA and the McKenzie TSH bioassay. Bioassay values, especially for plasmas, tended to be higher than those obtained by RIA. The mean ratio of bioassay to RIA values for the limited number of samples tested so far is about 1.5 +/- 0.2. Therefore, it does not appear correct to express rat TSH values obtained by RIA in biological potency units; they should only be given in terms of weight equivalents of an international rat reference preparation.