The thyroid gland is the only source of thyroid hormone production. Thyroid hormone is essential for growth and development, and is of special importance for the development of the central nervous system. It was for that reason that neonatal screening on congenital hypothyroidism was introduced and is now performed in many countries. Defects in thyroid hormone production are caused by several disorders in hormone synthesis and in the development of the thyroid gland (primary hypothyroidism) or of the pituitary gland and hypothalamus (central hypothyroidism).This paper describes defects in the synthesis of thyroid hormone caused by disorders in the synthesis or iodination of thyroglobulin, leakage of iodinated proteins by a stimulated thyroid gland and the presence of abnormal iodoproteins, mainly iodinated albumin, in the thyroid gland and blood circulation. Circulating thyroglobulin and abnormal iodoproteins, as well as the breakdown products of these iodoproteins excreted in urine, are used for etiological diagnosis and classification. Moreover, our finding of an enzyme that catalyses the dehalogenation of iodotyrosines, which is important for iodine recycling and required for economical use of iodine, is also referred to.
JJ de Vijlder and MT den Hartog
Biosynthesis of iodothyronines in thyroglobulin occurs by oxidative coupling of two iodotyrosine residues catalyzed by thyroperoxidase. To study the mechanism of iodothyronine formation, iodine-free thyroglobulin was non-enzymatically iodinated and after removal of non-incorporated iodide, incubated with lactoperoxidase and glucose oxidase between pH 4 and 9. The amount of thyroxine (T4). 3,5,3'-tri-iodothyronine (T3), 3,3',5'-tri-iodothyronine (rT3) and 3,3'-di-iodothyronine (T2) formed was measured by radioimmunoassays after hydrolysis of thyroglobulin. T4 is synthesized out of two di-iodotyrosine (DIT) residues in thyroglobulin. The pH dependence of T4 formation fits the dissociation curve of the DIT phenoxy group (pKa 6.5). The formation of T2, synthesized out of two mono-iodotyrosine (MIT) residues, shows a quite different pH dependence. Below pH 6, T2 synthesis could not be observed, while above pH 7.4 a relatively large increase occurred. The values up to pH 8 fitted the dissociation curve of the MIT-phenoxy group with a pKa of 8.7. The gradual loss in enzymatic activity of peroxidase and oxidase in the reaction made the values obtained above pH 8 unreliable. The importance of the ionization of the phenoxy group for the coupling reaction was further consolidated by showing that the pH-dependent oxidation of 2-methoxy-phenol (guaiacol) had 50% maximal product formation at pH 7, a value concordant with pKa 7.0 for the ionization of the phenoxy group of this agent. T3 and rT3 synthesis followed mainly the ionization curve of the inner-ring hydroxyl group, indicating that this ring has the greatest influence on hormonogenesis. Since anion formation facilitates the removal of an electron under oxidative conditions, the pH dependence agrees with the involvement of phenoxy radicals in iodothyronine synthesis, a process that most likely also occurs in vivo since it is mainly T4 that is formed in thyroglobulin.
JJ de Vijlder, C Ris-Stalpers and T Vulsma
F Cools, AG van Wassenaer, JH Kok and JJ de Vijlder
OBJECTIVE: Evaluation of thyroid hormone response to a single administration of triiodothyronine (T3) early postnatally to premature infants of <30 weeks gestational age. DESIGN: A prospective clinical trial with historical control. METHODS: Ten infants born <28 weeks gestational age and ten infants born between 28 and 30 weeks gestational age were given 0.5 microg/kg T3 intravenously at 12 h after birth. The infants <28 weeks gestational age were also treated with thyroxine (T4; 8 microg/kg, once daily) during the first 6 weeks of life. Premature infants from a previous trial served as a matched historical control group. Analysis of variance for repeated measurements was performed. RESULTS: For the infants <28-30 weeks gestational age mean plasma T3 concentrations were significantly higher in the T3-treated group (P=0.027) for at least 2 weeks, whereas mean plasma levels of T4, free T4 and TSH were comparable. For the infants <28 weeks gestational age plasma T3 levels were also significantly different after correction for gestational age (P=0.0002), with either comparable or higher values in the T3-treated infants up to 56 days after injection of T3. Mean plasma free T4 levels were lower during the first 3 days and higher or comparable thereafter (P=0.0014), and TSH suppression was more evident in the T3-treated infants (P=0.003). CONCLUSION: A single administration of T3 to premature infants <30 weeks gestational age early postnatally results in a sustained increase of plasma T3 levels during the first weeks of life. In infants of 28-30 weeks gestational age this occurs without change in plasma free T4 levels, whereas in infants <28 weeks gestational age a transient decrease of plasma free T4 was present. The increase in plasma T3 is possibly caused by a T3-induced increase of type I deiodinase activity.
CH Konings, AS van Trotsenburg, C Ris-Stalpers, T Vulsma, BM Wiedijk and JJ de Vijlder
OBJECTIVE: Subclinical hypothyroidism occurs in a number of children with Down's syndrome (DS). The reason for the mildly elevated plasma thyrotropin (TSH) concentrations is not known. The present study investigated whether decreased TSH bioactivity plays a role in this phenomenon. DESIGN: A retrospective study of plasma specimens from DS children with mildly elevated plasma TSH concentrations and thyroid hormone levels within the reference range, using a TSH receptor-adenylate cyclase mediated bioassay. METHODS: Strain JP26 Chinese hamster ovary (CHO) cells, stable transfected with the human TSH receptor, were incubated with unfractionated plasma (1/10 diluted in hypotonic incubation medium) of 10 DS children with subclinical hypothyroidism and nine euthyroid children with insulin-dependent diabetes mellitus as controls. cAMP released in the incubation medium was measured by RIA. Mock-transfected CHO cells were used to correct for non-specific CHO response. WHO Second International Reference Preparation of human TSH was dissolved and diluted in pooled normal human plasma and simultaneously bioassayed to match patient and control results. RESULTS: Plasma TSH levels were slightly increased in DS (mean +/- S.D., 6.5+/-1.3 mU/l, reference range 0.4-4.0 mU/l). Plasma TSH levels for controls (1.3+/-0.4 mU/l) were within the reference range. Plasma thyroid hormone levels in patients and controls were normal, plasma TSH binding inhibitory immunoglobulin and thyroid peroxidase antibodies were negative. cAMP levels (corrected for non-specific CHO response) in DS patients (18.4+/-3.9 pmol/well) and in controls (14.3+/-1.3 pmol/well) did not significantly differ from cAMP levels generated by patient-TSH equivalent TSH standards (16.3+/-0.9 pmol/well). CONCLUSIONS: The present results demonstrate normal TSH bioactivity in plasma of DS children, indicating that subclinical hypothyroidism in these patients is of primary (thyroidal) origin.
AG van Wassenaer, JH Kok, FW Dekker, E Endert and JJ de Vijlder
OBJECTIVE: To investigate the effect on thyroid hormone metabolism of the administration of thyroxine to very preterm infants. DESIGN AND METHODS: Two hundred infants of less than 30 weeks gestation were enrolled into a randomized, double-blind, placebo-controlled trial. Thyroxine (T4) (at a fixed daily dose of 8 microg/kg birthweight) or placebo was started 12-24h after birth and discontinued 6 weeks later. Plasma concentrations of T4, tri-iodothyronine (T3), reverse T3 (rT3), TSH, and thyroxine-binding globulin were measured weekly during trial medication and 2 weeks thereafter. RESULTS: The T4 and the placebo group each comprised 100 infants. Antenatal, perinatal, and postnatal clinical characteristics were comparable in both groups. T4 and rT3 were significantly increased in the T4 group. TSH concentrations were depressed in the T4 group and T3 was significantly decreased, probably as a result of TSH depression. The T4/T3 and T4/rT3 ratios differed significantly between the two study groups. CONCLUSIONS: Daily T4 administration during the first 6 weeks after birth to infants of less than 30 weeks gestation prevents hypothyroxinemia, but decreases plasma T3 concentrations. Our finding possibly implies that very preterm infants should receive supplements of both T4 and T3.