Thyroid hormones (TH) play a pivotal role in the development and function of the mammalian brain. Patients with impaired thyroid hormone transport into the brain tissue or in the case of defective local thyroid hormone receptor (collectively referred to as thyroid hormone resistance) subsequently experience psychomotor disabilities. The DEEPTYPE registry has been established with the objective of intensifying the genotyping and, in particular, the neurological phenotyping of patients exhibiting deficiencies in either the thyroid hormone transporter (MCT8) or the thyroid hormone receptor alpha (THRα). The objective of this registry-based study is to enhance the diagnostic yield for MCT8 and THRα deficiencies by employing the serum fT3/fT4 ratio as a more sophisticated screening parameter. Furthermore, the investigators will study the genomic regulation of both genes and attempt to identify further coding and non-coding mutations that result in TH resistance. The patient registry DEEPTYPE will document the retrospective and prospective clinical data of identified children in a comprehensive manner. This will enable the identification of three key groups: (i) patients with non-coding mutations, (ii) patients with milder phenotypes presenting only with a subset of symptoms seen in both classic conditions, and (iii) patients who are ready for clinical trials.