the term u201c ct receptor binding affinity u201d is intended to mean binding affinity as measured by any methods known to those of skill in the art including inter alia those methods which measure binding affinity by a dissociation constant an inhibition constant or an ic 50 value the term u201c having a ct receptor binding affinity of at least one tenth the affinity of radioiodinated ct for said receptor u201d is intended to mean that the dissociation constant k d of the reagent is not more than ten times the k d of radioiodinated ct as measured in a ct receptor direct binding or competitive inhibition assay or that the or inhibition constant k i or ic 50 of the reagent is not more than 10 times that of radioiodinated ct as measured in a ct receptor competitive inhibition assay the term u201c having a ct receptor binding affinity equal to or greater than native ct or radioiodinated species of ct for said ct receptor u201d is intended to encompass reagents having a dissociation constant k d equal to or less than that of unlabeled ct or radioiodinated ct as measured in a ct receptor direct binding or competitive inhibition assay alternatively this term may be interpreted as encompassing reagents having an inhibition constant k i or ic 50 equal to or less than that of native ct or radioiodinated ct as measured in a ct receptor competitive inhibition assay in accordance with.