A thyroxine enzyme conjugate of the formula ##STR4## wherein: any one of the W groups can be X or an H of any one of the W groups may be replaced by X, wherein X is a bond or linking group;X is 0 or 1, being 0 when W1 is the linking group; A is an enzyme bonded at other than its active site; n is on the average in the range of 1 to the molecular weight by A divided by 2,000; ??1-4 are iodine; L is