Cross-Expression of Thymic and Parathyroid Hormone Receptors Supports the Hypothesis of a Parathyroid–Thymus Port System

The thymus and parathyroid glands share a common embryological origin from the third pharyngeal pouch, yet their potential morphological and functional interconnections remain insufficiently explored. We conducted a comparative study integrating immunohistochemistry (IHC) and SEM on human thymic tissue, parathyroid adenomas, and parathyroid tissue excised during thyroidectomy. IHC staining targeted Thymosin-α1, CaSR, and PTH1R, with semi-quantitative evaluation of staining intensity and distribution. SEM analysis was performed at multiple magnifications to assess stromal organization and microvascular relief. Non-parametric statistical tests (Kruskal–Wallis with Mann–Whitney post hoc comparisons) were applied to clinical and laboratory data across the three cohorts. Scanning electron microscopy (SEM) revealed convergent ultrastructural features between thymus and parathyroid, including reticular stromal meshes and vascular grooves suggestive of comparable microcirculatory organization. IHC demonstrated robust Thymosin expression in thymus, with heterogeneous/apical distribution in parathyroid tissue; CaSR showed strong membranous and cytoplasmic expression in parathyroid, but weak diffuse signal in thymus; PTH1R exhibited low-to-moderate expression in thymus and moderate heterogeneous expression in parathyroid, with apical accentuation in adenomas. Statistical analysis confirmed significant differences in ionized calcium, PTH, and anti-AChR titers among the three cohorts (all p < 0.001), while TSH and calcitonin did not differ significantly. Our findings strengthen the hypothesis of a morpho-functional parathyroid–thymus axis. The robust parathyroid expression of CaSR and PTH1R aligns with established roles in calcium–PTH homeostasis, while the novel detection of Thymosin in parathyroid tissue suggests an expanded functional repertoire. These results highlight a continuum between embryological proximity and adult tissue cross-talk, with potential clinical implications for parathyroid pathology and immune regulation.