Inconsistency of the tibia test for estimating growth hormone in crude pituitary extracts

The complement fixation immunoassay (CFIA) was used for quantitating growth hormone (GH) in crude anterior pituitary extracts from rats subjected to thyroidectomy with or without cortisol and exogenous GH administration. The results obtained from this study were compared with pertinent bioassay (tibia test) results or correlated with the pituitary acidophil cell counts. Whereas it has been reported that pituitary GH levels are normal by the tibia test at 2 weeks after thyroidectomy, the highly specific CFIA method showed an actual 87% decrement which correlated well with the reduction in acidophilis. In addition, the apparently normal content of GH after cortisol administration to thyroidectomized rats, as measured by the tibia test, was contradictory to the very low acidophil population and to the marked reduction in pituitary GH content as measured by the CFIA. Furthermore, theoretical tibia responses illustrate the inconsistency of the bioassay in different experimental conditions. If, as previously suggested, the content of thyrotrophin (TSH) in the crude pituitary extracts renders the bioassay of GH a dubious procedure, then the superiority of immunoassay is obvious.