In Vitro Parathyroid Hormone Release in Patients with Secondary Hyperparathyroidism

Background: The purpose of this study was to determine the precise endocrim. characteristics of parathyroid function in secondary hyperparathyroidism (sHPT).
Methods: We examined the effects of extracellular ionized calcium (Ca²⁺) varying from 0.5 to 2.0 mM on parathyroid hormone (PTH) release in parathyroid cell suspensions using a mid-regional PTH assay. Cells were obtained from 26 patients with sHPT who were divided into two groups according to the type of hyperplasia they exhibited, either nodular (n=16) or diffuse [n= 10). For compdrison, we also analyzed data from nine patients with primary hyperparathyroidism (pHPT; adenomas).
Results: Significant in vitro suppression of PTH release by Ca²⁺ was observed in the majority of subjects, regardless of the histologic abnormality. The pHPT group exhibited no significant relationship between clinical and in vitro data. In contrast, in the sHPT group (taken as a whole), suppression of PTH release by Ca²⁺ exhibited a plateau at a total serum calcium concentration of 2.5 mmol/L, and a parathyroid gland weight of 2 g.
Conclusions: These findings suggest that there is a curvilinear relationship in sHPT, but not pHPT, between the in vitro calcium sensitivity of parathyroid cells and total serum calcium, as well as gland weight. The in vitro calcium sensitivity in sHPT remains constant when the total serum calcium concentration exceeds 2.5 mmol/L, or when the gland weight exceeds 2 g.