Peroxidase Defect in Congenital Goiter with Complete Organification Block

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Abstract
A 14-yr-old girl with congenital goitrous hypothyroidism is presented. Thyroidal iodide was released by perchlorate, indicating a complete iodide organification block. Serum total thyroxine was 0.6 μg/100 ml. After iv administration of 131I, thyroid uptake of 42% was reached at 30 min. Subsequently the thyroidal 131I content fell with a half time of 0.76 day. Thyroid clearance was 811 ml/min and chromatographs of serum and urine samples indicated the presence of only free iodide. The thyroid tissue obtained at thyroidectomy established a slice/medium [131I] ratio of 37, which was abolished by KC1O4 but not by methimazole. Enzymatic studies performed on the tissue demonstrated very low peroxidase activity (3% of normal values), according to results in the tyrosine iodinase, triiodide, and guaiacol assays. Preincubation of the enzyme with hematin did not increase activity. Neither digitonin nor deoxycholate + trypsin + acetone produced soluble enzyme. Catalase activity was normal and cytochrome c reductase, a possible source of endogenous thyroidal H2O2, was actually augmented. Leukocyte peroxidase activity was normal. Thyroglobulin was found to be the major soluble protein component of the thyroid by starch gel and polyacrylamide electrophoresis. The iodine content of native thyroglobulin was 0.53 pg per mg of protein and 0.56% iodine content was obtained by in vitro chemical iodination. However, enzymatic thyroglobulin iodination using human or bovine thyroid peroxidase failed to incorporate iodine into the protein.

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