Clearance of circulating desialylated thyroglobulins in the rat

Canine and rat thyroglobulins were labeled with 125I either in vitro or in vivo, and were utilized for plasma clearance studies performed with rat. The half-life of physiologically radioiodinated asialothyroglobulins was about 6 min, while that of chemically radioiodinated asialothyroglobulin was about 12 min. No marked species difference was observed in this clearance. The label which had disappeared from the blood was recovered mainly in the liver, and this uptake was blocked by the simultaneous injection of desialylated orosomucoid but not by native orosomucoid. Radiolabeled monoiodotyrosine, diiodotyrosine, triiodothyronine and thyroxine were detected in the liver 17 min after intravenous injection of asialothyroglobulin labeled with 125I in vivo, suggesting the possible production of thyroid hormones in extrathyroidal tissues.