On the Mechanism of the McKenzie Bioassay¹

Abstract

A number of substances including thyroglobulin and thyroxine (T4) affect the McKenziebioassay for thyroid stimulators. In part, this action results from enhanced clearance of T4 radioactivity from the circulation of the assay mouse. This was demonstrated bothin intact and in thyroidectomized T4-maintained mice. Thyroglobulin also acts by limiting the capacity of the thyroid to respond to stimulation. Since a substantial portion of the bloodradioactivity measured in the McKenzie assay consists of iodide, the assay is also subject to the influence of factors affectingiodide clearance. However, the iodide fraction of total blood radioactivity did not change after thyrotropin (TSH) stimulation and only slightly after stimulation by the longacting thyroid stimulator (LATS).The assay is also affected by the amount of T4given to assay animals during preparation for the McKenzie assay. This is extremely variable in the many modifications of the original assay procedure which are in common use. Six variants of the...