PTH1R and hyperparathyroidism: Second, given that recombinant human PTH (teriparatide, Forteo ®) is currently used as a bone anabolic agent for osteoporotic patients and that PTH and PTHrP share the common receptor PTH1R, increased PTH levels in the circulation due to rhPTH treatment or hyperparathyroidism will potentially increase MDSCs in the blood and reduce T-cell immunity.