Immunoassay methods and reagents for the specific quantification of thyroxine in a test sample are disclosed employing antibodies prepared with thyroxine derivatives of the formula: ##STR1## wherein P is an immunogenic carrier material and X is a linking moiety. The present invention also describes the synthesis of unique labelled reagent of the formula: ##STR2## wherein Q is a detectable moiety and W is a linking moiety, preferably fluorescein or a fluorescein derivative.