Bifidobacterium longum, TNF<sup>+</sup> T cells, and Th17 cells were mediators of bone loss in primary hyperparathyroidism patients, while Bifidobacterium longum supplementation caused PTH to expand TNF<sup>+</sup> T cells and Th17 cells and induce bone loss in mice. Here, PTH is linked to primary hyperparathyroidism.