The Reverse Sigmoid Ca-PTH Relationship is Not Symmetrical

We present a new, two-pool, linear, time-varying model to describe the short term dynamics of the Ca-PTH axis. We use an asymmetrical reverse sigmoid PTH secretion rate function in the model. This results in an asymmetrical reverse sigmoid relationship between plasma Ca⁺⁺ and plasma parathyroid hormone (PTH) concentrations (Ca-PTH relationship). In the first validation of this kind, with parameters estimated separately based on each subject’s hypocalcemic clamp test data, we successfully test the model’s ability to predict the same subject’s induced hypercalcemic clamp test responses. We then show that the conventional symmetrical reverse sigmoid Ca-PTH relationship is deficient.