Iodinated proteins in normal human thyroid gland. Thyroid albumin.

Iodinated proteins in normal thyroid tissue extracts were analyzed immunochemically, immunohistochemically and physicochemically after isoelectric focusing. An albumin-like protein ranging from pI 4.80 to 5.21 was found in normal human thyroid tissue extracts, and its pI range was not compatible with that of serum albumin ranging from pI 4.85 to 6.16. An immunodiffusion study suggests that T3 loosely combines with the albumin-like protein, and that this compound is so-called thyroid albumin. Amino acid analysis revealed a difference in the basic amino acid composition of thyroid albumin and serum albumin, and our immunohistochemical study showed that thyroid albumin and T3 were localized in follicular epithelial cells, but not in the colloid. Based on these results, thyroid albumin may be described as consisting of T3 and an albumin-like protein which is distinguishable from serum albumin and distinct from thyroglobulin immunochemically. It appears that a low molecular weight hormone like T3 can be detected when it combines with a protein molecule. Recently, DeGroot et al. (1975) suggested a collateral pathway of hormone metabolism through the thyroid albumin in thyroid follicular cells. Our results seem to support the existence of this pathway.