Validation of a novel high-sensitivity radioimmunoassay procedure for measurement of total thyroxine concentration in psittacine birds and snakes

Objective-To validate a novel high-sensitivity radioimmunoassay (RIA) proce dure developed to accurately measure the relatively low serum total thyroxi ne (T-4) concentrations of birds and reptiles and to establish initial refe rence ranges for T4 concentration in selected species of psittacine birds a nd snakes. Animals-56 healthy nonmolting adult psittacine birds representing 6 species and 42 captive snakes representing 4 species. Procedure-A solid-phase RIA designed to measure free T4 concentrations in d ialysates of human serum samples was used without dialysis to evaluate tota l T4 concentration in treated samples obtained from birds and reptiles. Ser um T4 binding components were removed to allow assay of undialyzed samples. Assay validation was assessed by determining recovery of expected amounts of T4 in treated samples that were serially diluted or to which T4 was adde d. intra- and interassay coefficient of variation (CV) was determined. Results-Mean recovery of T4 added at 4 concentrations ranged from 84.9 to 1 15.0% and 95.8 to 119.4% in snakes and birds, respectively. Intra- and inte rassay CV was 3.8 and 11.3%, respectively. Serum total T4 concentrations fo r 5 species of birds ranged from 2.02 to 768 nmol/L but ranged from 3.17 to 142 nmol/L for blue-fronted Amazon parrots; concentrations ranged from 0.2 1 to 6.06 nmol/L for the 4 species of snakes. Conclusions and Clinical Relevance This new RIA method provides a commercia lly available, accurate, and sensitive method for measurement of the relati vely low serum T4 concentrations of birds and snakes. Initial ranges for th e species evaluated were established.