Pituitary Pars Intermedia Dysfunction (PPID) is the most common neurodegenerative endocrine disease in horses older than 15 years. Dental diseases and especially periodontitis occur regularly in older horses. Although PPID in geriatric horses and dental disease in all age groups are well described, possible associations between this endocrine disorder and pathologic changes in equine dental structures have not yet been established. In the case of PPID, cross-connections to immune dysregulation, disturbed collagen synthesis and delayed healing processes have been proven. Therefore, the aim of the published studies was to first identify macroscopic and then histological changes in the gingiva and subgingival periodontal structures associated with PPID. In the first part of the study, 14 horses with clinical signs of PPID and adenoma in the pars intermedia of the pituitary gland and 13 control horses that showed neither clinical signs nor PPID-associated histological changes in the pituitary gland were included. PPID-affected horses (26.9 ± 0.73 years) were significantly older than controls (20.0 ± 1.24 years; p = 0.0006). For the first time, macroscopic changes in the gingiva of horses with PPID compared to controls were described. The significantly more frequent occurrence of changes in the gingival structure with a voluminous, irregular appearance (p < 0.0001), an irregular gingival course (p = 0.04) and increased evidence of deepened gingival sulci (p = 0.004) indicate a predisposition of PPID-affected horses for periodontitis. However, endocrinologically caused pathomechanisms that can explain the weakened gingival tissue structure in PPID still have to be identified in the future. The second publication is a morphometric-descriptive case-control study. A total of 145 tooth locations from 10 PPID-affected horses (27.3 ± 2.06 years) were compared with 147 tooth locations from 10 control horses (21.4 ± 4.12 years; p < 0.001). Histological parameters were leucocyte infiltration, keratinization of the gingival epithelium (oral gingival epithelium, sulcus epithelium and junctional epithelium), blood vessel supply to the periodontium and structure of the cementum. The distribution and localization of gingival leukocyte infiltrates in PPID-affected horses was more frequently multifocal to confluent (p = 0.002) and extended into deeper parts of the periodontium, sometimes down to the subgingival periodontal ligament (PDL). Older animals of both groups showed a higher prevalence (PPID: OR 1.66; controls: OR 1.15) of high-grade leukocyte infiltration in the periodontal ligament. However, age was more important than PPID status for the greater accumulation and number of leukocyte infiltrates in the PDL. The cementum at interdental positions showed four times more irregularities in PPID-affected horses than in controls, predisposing to food impaling, diastema formation and periodontal disease. For the first time horses suffering from PPID were compared with healthy old controls in histological parameters of the equine periodontium. Further characterization of leukocytes would be required to assess in detail their possible role in destructive periodontal processes in PPID-affected horses. Overall, it can be concluded that horses suffering from PPID show changes both macroscopically and histologically compared to healthy controls, which in themselves can be predisposing factor for periodontal disease. Overall, however, age has a greater influence than PPID status on the teeth and periodontal structures of the horse.