Studies on an Iodinated Protein in the Serum of Subjects With Cancer of the Thyroid*†

Tumors of endocrine glands may possess functional activity and secrete “normal” hormones or substances with similar physiologic activity but different in chemical structure. Cancers of the thyroid in the absence of any normal tissue may maintain a subject in an euthyroid state. It has been demonstrated that some cancers of the thyroid, especially the metastatic variety, secrete thyroxine and triiodothyronine (1). Moreover, an iodinated substance, presumably protein in nature and not present normally, has been found in the blood of some patients with cancer of the thyroid (1). This material was shown to have an electrophoretic mobility at both pH 8.6 and pH 4.5 similar to that of serum albumin. It was not dialyzable and has a sedimentation constant S20, w =4.2 (2). This substance, designated compound X, could be hydrolyzed by trypsin or alkali and yielded monoiodotyrosine and a very small amount of thyroxine. The present study was undertaken to investigate in more detail some of the properties of compound X labeled with I131 and, in particular, to evaluate its relationship to serum albumin.