Estimation of thyroidal radioiodide clearance in the domestic fowl (Gallus domesticus).

: A method for estimating thyroidal clearance of radioiodide from the blood plasma of the domestic fowl is described. It differed from published methods in a number of ways. (1) It utilized total thyroid radioiodide concentration (IT) and did not require the use of goitrogens or the separation of free and protein-bound components. (2) Radioiodide was injected intravenously rather than by other routes. (3) Plasma radioiodide concentration (IB) was determined from several serial samples from each bird rather than once only. (4) The method allowed the clearance constant (kappa; microliter/min per mg) to be estimated for individual birds, rather than from groups, thereby enabling effective replication for comparative experiments. The constant was estimated from the model dIT/dt = kappaIB, measurements being made within 120 min after injection to ensure that exit of radioiodide from the thyroid was negligible. The improved method resulted in estimates of clearance constants which agreed well with published findings.