PTH and cyclic hematopoiesis: However, when CH treatment, the interaction term between intact PTH and CH (IT iPTH*CH), the interaction term between intact PTH and Ca (IT iPTH*Ca), and the interaction term between CH and Ca (IT CH*Ca) were added to the model as independent variables, only intact PTH, IT iPTH*CH, and IT iPTH*Ca were indicated as significant factors determining the circulating whole PTH levels.