In patients with primary hyperparathyroidism, the inverse relationship between serum PTH (and parathyroid cell number) and serum calcium still exists, although the relationship is altered and occurs at a different level of serum calcium. In other words, PTH secretion can be suppressed or kept constant by an increase in extracellular fluid calcium concentration above a critical level, although the sensitivity of the parathyroid glands to changes in serum calcium is altered and set at a different level. This occurs in the majority of patients with parathyroid gland adenomas. In patients with parathyroid hyperplasia, the set point for PTH release may be normal, but the mass of parathyroid tissue is increased and so the result is a relatively greater circulating PTH concentration for any serum calcium concentration.
