Metabolism of99mPertechnetate by the Thyroid Gland of the Rat1

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Abstract
The thyroidal metabolism of 99mpertechnetate (TcO4-) was studied after administration of the isotope to rats in vivo or after incubation of excised rat thyroids with the isotope in vitor. In both types of experiment, thyroids actively concentrated TcO4-. Electropho-retic and chromatographic analyses of homogenates prepared from thyroids after exposure to the isotope revealed both free TCO4- identical in mobility to the original isotope, and a new immobile form of the isotope which remained at the origin (origin material, OM). Formation of OM was not inhibited by conventional doses of inhibitors of the organisation of iodide, and OM often comprised as much as 70% of total accumulated technetium (Tc). OM was resistant to digestion by Pronase and 6N HCl; exposure to 2N NaOH resulted only in the release of free TcO4-. Thyroid/medium (T/M) and thyroid/serum (T/S) concentration ratios for free TcO4- were increased by feeding low iodine deits or by chronic administration of propylthiouracil (PTU) in Purina Chow. Ratios were decreased by chronic thyroxine (T4) administration, diets rich in iodine, incubation of excised thyroids in media enriched with stable iodide and by chronic administration of PTU in a low iodine diet. In all experiments, concentration ratios for free TcO4- were qualitatively similar to those expected if iodide were the substrate instead. In 2 studies in which concentration ratios for both 99mTcO4- and I131- were simultaneously determined, ratios of the T/M values for TcO4- and I- were not constant and varied by as much as 50% in thyroids from animals of different experimental groups. It is concluded that measurements of TcO4- transport may be used as a qualitative, but not quantitative, index of alterations in the activity of the thyroidal iodide transport mechanism. In view of the invariable generation of a chromatographically and electrophoretically immobile product from free TcO4-, even qualitative studies require the application of separative techniques.