Resetting of parathyroid hormone secretion after vitamin D3 treatment in hypoparathyroidism and after parathyroid adenectomy in primary hyperparathyroidism

The relationship between parathyroid hormone (PTH) secretion and extracellular calcium (Ca) level is reciprocal causality. The equilibrium operating point determines basal PTH secretion rate and basal extracellular Ca level. We studied how this equilibrium was achieved in the subjects with decreased PTH secretion or decreased parathyroid glands number. Basal/maximum ratio of serum PTH, which reflects the basal secretory state of parathyroid glands, was increased in 9 hypoparathyroid patients treated with vitamin D3 (VD3) [7 patients with idiopathic hypoparathyroidism] and in seven of nine parathyroid adenectomized patients. There was a negative correlation between the ratio and basal serum Ca level in the patients with IHP after VD3 treatment (r = 0.7167, P < 0.05) and in the patients after parathyroid adenectomy (r = 0.7760, P < 0.05). The regression curves in these two groups coincided regardless of the difference in maximum PTH secretion rate, which suggested that the basal secretory state of parathyroid glands was determined by extracellular Ca level in a similar manner in these subjects. There was a sigmoidal relationship between basal/maximum ratio of serum PTH and basal serum Ca level, when the data were collected from 15 hypoparathyroid patients before or after VD3 treatment, 9 parathyroid adenectomized patients, and 10 normal subjects (r = 0.9057, P < 0.001). This sigmoidal curve is thought to represent the fundamental relationship between the basal secretory state of parathyroid glands and extracellular Ca level.