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) procedure developed to accurately measure the relatively low serum total thyroxine (T-4) concentrations of birds and reptiles and to establish initial reference ranges for T4 concentration in selected species of psittacine birds and 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 dialysates of human serum samples was used without dialysis to evaluate total T4 concentration in treated samples obtained from birds and reptiles. Serum 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 added. intra- and interassay coefficient of variation (CV) was determined. Results-Mean recovery of T4 added at 4 concentrations ranged from 84.9 to 115.0% and 95.8 to 119.4% in snakes and birds, respectively. Intra- and interassay CV was 3.8 and 11.3%, respectively. Serum total T4 concentrations for 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 0.21 to 6.06 nmol/L for the 4 species of snakes. Conclusions and Clinical Relevance This new RIA method provides a commercially available, accurate, and sensitive method for measurement of the relatively low serum T4 concentrations of birds and snakes. Initial ranges for the species evaluated were established.