Circulating Thyroglobulin Transcytosed by Thyroid Cells Is Complexed with Secretory Components of Its Endocytic Receptor Megalin*

After its endocytosis from the colloid, some thyroglobulin (Tg) is transcytosed intact across thyrocytes, accounting in part for its presence in the circulation. We previously showed that megalin (gp330), an endocytic Tg receptor, mediates apical to basolateral Tg transcytosis. Here we investigated whether a portion of megalin remains combined with Tg after its transcytosis, using studies with cultured thyroid cells and in vivo observations....