Release of thyroid hormone from circulating thyroglobulin in the rat.

Under normal conditions, a small amount of thyroglobulin (Tg) exists in peripheral blood. However, the fate of circulating Tg is unclear. In the present study, in vivo labelled rat Tg was injected iv into rats whose thyroids had been blocked with KI to determine whether circulating Tg released thyroid hormone by hydrolysis in extrathyroidal tissues. Radiolabelled Tg was obtained from thyroid of rats injected with 125I 24h before sacrifice, and subsequently purified by ammonium sulphate precipitation. The plasma samples were obtained from tail veins or by cardiac punctures at various times following injection of [125I]rat Tg. The radioactive samples were separated into iodoprotein, iodoaminoacid and iodide fractions using columns of anion and cation exchange resins. The per cent radioactivity of the iodoprotein, iodoaminoacid an iodide fractions, respectively, was 91.2, 3.8 and 5.2 at 15 min and 66.9, 17.4 and 15.4 at 20 h after injection. In the iodoaminoacid fractions, the presence of T4, T3, MIT and DIT was defined by further fractionation using a Sephadex G-25 column. At 20 h after injection, more than 75% of the radioactivity of the iodoaminoacid fraction was found to be incorporated in T4 and T3. It is concluded that circulating Tg is hydrolyzed in extrathyroidal tissues and that thyroid hormone is released into the circulation, but the amounts of T4 and T3 released are not physiologically significant.