Paediatric thyroid disorders: new insights.

random monoallelic expression. Thus, the hypothesis of non-Mendelian inheritance remains valid, and the search for other mechanisms ensues. They do not exclude a complex situation in which both Mendelian and non-Mendelian mechanisms are at play. In the clinical part of this special issue, thyroid disorders associated with well-known genetic syndromes are carefully analyzed and reviewed. First, hypothyroidism is a typical feature of pseudohypoparathyroidism (OMIM 103580), but due to its mild course, it remains to be determined whether compensated hypothyroidism is present at birth or develops later in life, and at what time it should be looked for. A large retrospective study presents clear answers. In a second work, the clinical features of thyroid disorders in the Rett syndrome (OMIM 613454) are outlined. Finally, a detailed update is given on mechanisms of non-autoimmune causes of hypothyroidism in Down’s syndrome (OMIM 190685). All three papers provide important information and updates on the current state of knowledge and on future directions of research. Many clinical problems in the field of thyroid disease in paediatric patients remain unresolved. Therefore, the 2015 ‘special topic issue’ of Hormone Research in Paediatrics focuses on new developments and new knowledge in this field, and some of the papers selected will add new insights while others provide new hypotheses. Although this special issue focuses on clinical problems, the first article adds an important piece of knowledge on a long-standing question in the context of sporadic rather than inherited congenital hypothyroidism due to thyroid dysgenesis. The fact that monozygotic twins are discordant for thyroid dysgenesis led the authors to hypothesize that rather non-Mendelian than Mendelian mechanisms could be involved in thyroid dysgenesis. They performed whole exome sequencing in lymphocytes of three monozygotic twin pairs discordant for thyroid dysgenesis without evidence of differences in the protein-coding genome using current technology. Although their initial hypothesis of possible early somatic mutations in the affected monozygotic twins was not confirmed, thus presenting ‘negative’ results, the data deserve special consideration. On the one hand, the authors themselves point to the fact that somatic mutations have not been excluded as the cause of thyroid dysgenesis; on the other hand, they suggest another genetic mechanism: Published online: March 20, 2015 HORMONE RESEARCH IN PÆDIATRICS