Sigmoidal curve shift in idiopathic hypoparathyroidism and pseudohypoparathyroidism.

The sigmoidal curves plotting serum parathyroid hormone (PTH) against serum Ca in primary hyperparathyroidism and secondary hyperparathyroidism due to renal failure deviate to the right. We previously found the leftward curve shift in PTH-deficient hypoparathyroidism. In the present study, we investigated the curve shift in pseudohypoparathyroidism (PHP) with secondary hyperparathyroidism due to target organ resistance to PTH. In renal failure the sigmoidal curves move to the left after vitamin D3 treatment. We also examined the effect of vitamin D3 on the curve shift in pseudohypoparathyroidism (PHP) and idiopathic hypoparathyroidism (IHP). Before vitamin D3 treatment, the sigmoidal curve deviated to the left in both types of hypoparathyroidism. After vitamin D3 treatment it moved to the right. These results indicate that vitamin D3 and/or extracellular Ca modify the relationship between PTH and Ca dynamics even in hypoparathyroid disorders with decreased or increased maximum serum PTH. Following vitamin D3 treatment, the point plotting baseline serum PTH against baseline serum Ca moved to the right at first in accordance with the rightward shift of the sigmoidal curve and then the point moved downward in PHP or downward in IHP. These changes suggest that vitamin D3 resets PTH secretion at a higher extracellular Ca level at first and then suppresses it in a time-dependent manner. 1, 25(OH)2D3 and/or extracellular Ca may be the determinant factors of the sigmoidal curve shift in hypoparathyroid disorders. Mechanisms other than the Ca sensing system error may contribute to the curve shift.