Early Thyroid Injury and Parathyroid Resistance Following Ionizing Radiation: Evidence of Paradoxical Hsp-90 Downregulation

Objective Ionizing radiation (IR) is widely used in the treatment of head and neck malignancies; however, its early effects on adjacent endocrine tissues remain insufficiently characterized. This study aimed to investigate early post-irradiation structural and molecular alterations in the thyroid and parathyroid glands, with a particular focus on HSP-90-mediated proteostasis. Materials and Methods Male Wistar-Hannover rats were assigned to control or irradiation groups and exposed to a single 18 Gy dose of IR to the head and neck region. On day 21, thyroid and parathyroid tissues were evaluated histopathologically and immunohistochemically for morphological changes and HSP-90 expression. Results Irradiated thyroid tissue exhibited significant vascular congestion (p = 0.031), inflammatory infiltration (p = 0.040), and epithelial desquamation (p = 0.006). Notably, HSP-90 expression in thyroid follicular cells demonstrated a paradoxical downregulation , being significantly reduced compared with controls (p = 0.011). In contrast, parathyroid glands showed preserved architecture and stable HSP-90 expression (p = 0.914). Conclusion A single high-dose IR exposure induces early microstructural injury in the thyroid while paradoxically suppressing HSP-90-mediated stress responses, whereas the parathyroid gland appears relatively resistant at this stage. These findings highlight organ-specific differences in endocrine radiosensitivity and suggest that early impairment of proteostasis may contribute to radiation-induced thyroid vulnerability.