Fig. 4. Plot of the least-squares regression lines for a three-parameter exponential fit describing the relationship of serum PTH to serum 25(OH)D for normal weight, overweight, and obese women in our study cohort. The equations are identical in form to that employed in Fig. 3. The general similarity of the three curves is evident. (Copyright, Robert P. Heaney, 2005. Used with permission.)