THE TEST OF OVERLOADING OF L-DIIODOTYROSINE (DIT) IN THE SCREENING OF IODOTYROSINE DEHALOGENASE DEFICIENCY

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Abstract
The hereditary nature of goitrous hypothyroidism due to the iodotyrosine dehalogenase deficiency (IDD) was suspected. In order to demonstrate a partial deficiency in the relatives and parents of the patients, the peripheral deiodination of labeled L-diiodotyrosine (DIT) was studied using urinary elimination. The usual procedure consists of a standard test which was performed by i.v. injection of a tracer dose of radioactive DIT. In order to increase the sensitivity of the method a test of overloading was carried out by i.v. administration of 20-25 mg of highly purified [127I]L-DIT labeled with 50 .mu.Ci of 125I in adults and a lower dose in children. After injection the quantity of non-deiodinated DIT was measured in urinary samples at intervals of 0-2, 2-6 and 6-24 h. In 15 normal subjects the urinary elimination of DIT in 6 h varied from 1.9-23.9% of the injected dose. In 8 goitrous hypothyroid patients, 52-79.9% was eliminated, most of the excretion occurring during the first 2 h. In 22 relatives of the patients it was 6.9-43% of the dose, and in 1 of the patient''s relatives who was clinically normal it was 70.9%. A statistical study showed that the most significant characteristic was the percentage of DIT excreted during the first 2 h. Elimination in the normal group was 11.4% (SD 5.8) while that of the parents and relatives of goitrous hypothyroid patients was 20.4% (SD 12.5). The 2 groups are statistically different but with a poor P-value (P < 0.05). Discriminant analysis permitted classification of each relative with respect to the control group: among 23 parents and relatives, 15 differed from the controls and were suspected of representing a partial IDD. The latter can be considered as heterozygotes, and according to the results of the proposed DIT overloading test the transmission of congenital IDD is determined as an autosomal recessive trait. However, in 1 family, the hypothesis of the variable expressivity of a genetic dominant trait must be considered.

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