Primary hyperparathyroidism (PHPT) may induce bone loss according with the composition of the gut microbiota (GM), and particularly, on the presence of intestinal bacterial that induce T helper 17 differentiation. We will evaluate GM composition and evaluate how GM modulates immune system in patients affected by PHPT with or without skeletal involvement. Furthermore, we will unravel the causal relationship between GM composition and T cells activation. Upon success, HYPOGEUM will show that GM sequencing is a screening tool to identify PHPT that will lose bone, suggesting novel strategies with antimicrobial treatments to prevent bone loss. HYPOGEUM will yield essential data to understand and prevent skeletal complications associated with PHPT.