Pituitary neuroendocrine tumors (PitNETs), formerly known as adenomas, are heterogeneous neoplasms that can result in significant clinical morbidity. The molecular pathogenesis of these tumors has been linked to mutations in genes such as GNAS, MEN1, and USP8, with their roles in tumor development well characterized. However, the potential contributions of the phosphatase and tensin homolog (PTEN) and microsatellite instability (MSI)-related genes remain insufficiently elucidated. This study aimed to investigate the immunohistochemical expression profiles of PTEN, phosphorylated protein kinase B (pAKT1), phosphorylated ribosomal protein S6 (pRPS6), and DNA mismatch repair (MMR) proteins in PitNETs. In this retrospective analysis, archived formalin-fixed, paraffin-embedded tumor samples from 154 patients were stained for PTEN, pAKT1, and pRPS6, and H-scores were calculated based on the three most intensely stained high-power fields. Additionally, MMR protein expression (PMS2, MLH1, MSH2, and MSH6) was assessed to evaluate MSI status. Gonadotroph tumors were the most common subtype (45.5%). Statistical analysis revealed a significant loss of PTEN expression in lactotroph tumors, along with a sex-based association with tumor type. Notably, pRPS6 expression was significantly elevated in tumors exhibiting PTEN loss. No cases showed loss of MMR protein expression, indicating MMR proficiency across the cohort. Our findings suggest that dysregulation of the PTEN pathway may contribute to lactotroph tumorigenesis, underscoring its potential as a therapeutic target. In contrast, despite its established role as a tumor-agnostic biomarker, MMR status does not appear to hold clinical relevance for alternative treatment strategies in PitNETs.