Recovery of Thyroid Hormone Biosynthesis after a Single Injection of Methylthiouracil as Judged by Thyroidal Radioiodine Uptake, Thyroidal Protein-Bound Iodine, Thyroidal Radioiodinated Amino Acids and Thyroglobulin in the Rat

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In an attempt to assess the synthesis of thyroid hormone in the rat after a single large dose of methylthiouracil, radioiodine was injected 0 to 164 hrs. after administration of 20 mg methylthiouracil (MTU). Thyroidal radioiodine uptake, thyroidal PB 131I, thyroidal radioiodinated amino acids and thyroidal content of thyroglobulin were measured. When radioiodine was injected at 0 time, 80% of total thyroidal radioiodine was in organic form at 24 hrs., while thyroidal radioiodine uptake was extremely low because of rapid loss of 131I- from the blood in MTU treated animals. Thyroidal PB 131I does reflect MIT but not T4 and T3. On the other hand, when radioiodine was injected 4 hrs. before autopsy, all thyroidal radioiodine uptake, thyroidal PB 131I and thyroidal T4 and T3 returned to normal between 3 and 4 days, indicating a good correlation among different thyroid parameters. DIT/MIT ratio, however, showed some delay in its return. Thyroidal content of thyroglobulin decreased when thyroid hormone synthesis was blocked by MTU, but returned to normal when synthesis of thyroid hormone was normal. It is suggested that methylthiouracil acts on the thyroid in two ways; one, blocking oxydative enzyme and the other, depressing thyroglobulin content.
社団法人日本内分泌学会の論文