Objectives: The main objective of this study was to observe differences in the conversion rate of thyroid hormone parameters free thyroxine (fT4) to triiodothyronine (fT3) between patients with stages 2 and 4 of chronic kidney disease (CKD). The secondary objective was to compare thyroid hormone levels between subgroups of CKD stage 2 and 4, specifically comparing patients who received thyroid hormone replacement therapy with levothyroxine and those who did not receive any thyroid hormone medication. Patients and methods: In this retrospective observational study, clinical data of patients admitted to the REGIOMED Hospital Coburg and Hildburghausen in Bavaria, Germany were included. CKD categorizations occurred based on the international statistical classification of diseases and related health problems (ICD) code from the hospitals Orbis System. The chosen time period of inclusion was set to 1. January 2020 to 31. July 2023. The data was pseudonymized for further use. The total study population included 183 patients, after applying the exclusion criteria to the initial 220 participants. The statistical analysis was carried out with JMP pro 17 program. Results: This study found that patients with CKD stage 4 had significantly lower fT3 levels and a reduced fT3/fT4 ratio compared to those with CKD stage 2, indicating an impaired conversion of fT4 to fT3 in the later stages of CKD. This decrease was observed even among patients receiving levothyroxine therapy. TSH and fT4, however, did not show any significant changes amongst groups and subgroups and remained within reference range. Additionally, the study revealed lower SPINA-GD levels in CKD stage 4 patients, further supporting the hypothesis of diminished peripheral deiodinase activity in these patients. Conclusion: The results of this research revealed significant impairment in the conversion of fT4 to fT3 in patients with advanced CKD. The decreased fT3 levels, despite levothyroxine therapy, suggest that new treatment methods may be needed for managing thyroid dysfunction and counteracting the associated increased risk for mortality in this patient population. Therefore, further research is necessary to explore the directionality of disease occurrence and to find optimal therapeutic approaches. Additionally, clinical implications of nonthyroidal illness syndrome within the different stages of CKD need to be further investigated as well as the distinct degree of renal dysfunction, leading to thyroidal abnormalities.