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One mechanism for the preservation of iodine within the body is the deiodination of monoiodotyrosine and diiodotyrosine and reuse of the released iodine. The elucidation of this mechanism has a long history beginning in 1952 with the extraction of an iodotyrosine dehalogenase from thyroid tissue. The enzyme was further characterised in 1970 and in 2001 serial analysis of gene expression led to the identification of four genes, one of which was given the name dehalogenase 1 (DEHAL1). Now four patients who had severe goitrous hypothyroidism from infancy or later childhood have been shown to have DEHAL1 mutations (José C Moreno and colleagues. New England Journal of Medicine 2008;358:1811–8; see also editorial, ibid: 1856–9). Two had had normal neonatal thyroid screening and two had not been screened. Hypothyroidism had been diagnosed clinically in infancy (two sisters), at the age of 18 months, and at 8 years. Two patients were treated late and had learning difficulties. Serum concentrations of diiodotyrosine and uptake of 123I were increased in the two patients tested. All four patients had severely reduced in vitro iodotyrosine dehydrogenase activity. Three different mutations in DEHAL1 were detected, one in the sisters and one in each of the other two patients. Infants with DEHAL1 mutations may not be picked up by neonatal screening because their …

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