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Peroxidase Defect in Congenital Goiter with Complete Organification Block

Thyroid Study Unit, Department of Medicine, University of Chicago Chicago, Illinois Departments of Pediatrics and Radiology, Section of Nuclear Medicine, State University of New York, Downstate Medical Center Brooklyn, New York

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.

Supported in part by United States Public Health Service Grants AM-13,377, AM-15,070, and RR-55. Presented in part at the forty-seventh meeting of the American Thyroid Association, October 12–15, 1971, Birmingham, Alabama.

¹ United States Public Health Service International Postdoctoral Research Fellow and Investigator of the Atomic Energy Commission of Argentina.

Received April 3, 1972.

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