Thyroglobulin (TG), one of the largest known eukaryotic proteins (Mr 660.000, sedimentation coefficient 19S), plays a central role in thyroid hormonogenesis. TG presents several characteristics of special interest: (i) it represents the major biosynthetic product of thyroid follicular cell1; (ii) under denaturing conditions it can be dissociated into identical half molecules (Mr 330.000, sedimentation coefficient 12S)2; (iii) its native structure is essential for an efficient thyroid hormone production3. TG is synthesized on membrane bound polysomes4 and core-glyco-sylated in rough endoplasmic reticulum5. Along its migratory pathway TG passes through the Golgi apparatus in which carbohydrate processing is accomplished and terminal sugars are added6, it is then secreted into the follicular lumen, where iodination takes place7. The ultimate fate of thyroglobulin is the reabsorption by pinocitosis, proteolytic degradation in the lysosomes and release of thyroid hormones in the blood stream.