Objective:To confirm the expression of parathyroid hormone(PTH) gene in mouse thymus,determine the localization of PTH protein in mouse thymus and explore whether thymus-derived PTH is regulated by the alteration of serum calcium levels. Methods:The difference of PTH gene and protein expression in parathyroid gland and thymus were compared between PTH knock-out mice 1α(OH)ase knock-out mice and their wild-type littermates at 2 weeks of ages by RT-PCR,real-time PCR and immunohistohemistry. Results:PTH gene and protein were undetectable in parathyroid gland and thymus in PTH knock-out mice,but were detectable in both organs in wild-type mice,despite the fact that the gene and protein expression of PTH was lower in the thymus than in the parathyroid glands. PTH was localized in the epithelium,but not in lymphocytes in thymus. PTH gene and protein expression levels were up-regulated in parathyroid gland and thymus in 1α(OH)ase knock-out mice with hypocalcaemia. Conclusion:The thymus is the another source of parathyroid hormone,in which the expression of PTH is regulated by the alteration of serum calcium levels.