Deficient thyroid peroxidase causing organification defect and goitrous hypothyroidism

Abstract

A 36-year-old male with congenital goitrous hypothyroidism is presented. Thyroidal iodide was released by Perchlorate (62%) indicating a iodide organification defect. Serum T₄ was 0.8 μg/dl, T₃ was 16 ng/dl and TSH level 190 μU/ml. Tests for autoantibodies against thyroid antigens were negative. Chromatographs of serial serum samples after a tracer dose of ¹²⁵I indicated only the presence of iodoalbumin (48.5%). At surgery the gland weighed 1,680 g and histologically the tissue showed marked heterogeinity. Total iodide content of the thyroid was very low (3.4 μg of l/g of tissue). The iodothyronines comprised only 8.1% of the iodocompounds in the pronase-digested homogenate of the gland, and the major components were the iodotyrosines (82.6%) with a very high DIT/MIT ratio (3.6). Enzymatic studies performed on the tissue demonstrated very low peroxidase activity (7–15% of normal values) according to results in the triiodide, guaiacol and tyrosine iodinase assays. Preincubation of the enzyme with hematin did not increase peroxidase activity. Catatase activity was normal and cytochrome c reductase, a possible source of endogenous thyroidal H₂O₂, was also within the normal range. Peroxidase was normally present in the saliva and in peripheral leucocytes. Thyroglobulin (Tg) was found to be the major soluble protein component of the homogenate although the iodine content of Tg was markedly reduced (0.16%). It is suggested that the congenital absence of peroxidase in this patient causes an organification block and also possibly impairs the coupling reaction within the thyroglobulin molecule.