Ratio of baseline level/maximum level of serum PTH in pseudohypoparathyroidism and primary hyperparathyroidism

The ratio of baseline level/maximum level of serum parathyroid hormone (PTH) is high in PTH-deficient hypoparathyroidism and it decreases after vitamin D3 treatment. There is a reversed sigmoidal relationship between the ratio and baseline serum Ca level. In this study, we further investigated the value of this ratio as a parameter of Ca-dependent changes of serum PTH in hyperparathyroid subjects. As in PTH-deficient hypoparathyroidism, the ratio in pseudohypoparathyroidism was high before vitamin D3 treatment and it decreased after 1,25(OH)2D3 treatment. The increased ratio may reflect the stimulated baseline PTH secretion from parathyroid cells perceiving the decrease in baseline extracellular Ca level. The points plotting the ratio against baseline serum Ca level were on the regression curve deduced from the data in PTH-deficient hypoparathyroidism. This result indicates that the relationship between the ratio and the baseline extracellular Ca level is unrelated to the variation in maximum secretion. The sigmoidal changes of serum PTH in patients with parathyroid adenoma were classified as follows. The first was with the upward and rightward curve shift, the second was only with the rightward curve shift, and the third was with the rightward curve shift, the increased minimum serum PTH, and the increased baseline/maximum ratio of serum PTH. These findings suggest that the decreased suppressibility of PTH secretion and the stimulated baseline secretion may develop without the increase in maximum secretion in some cases with parathyroid adenoma. In conclusion, the ratio of baseline level/maximum level of serum PTH may unfold a new aspect of secretion abnormality of parathyroid glands in several forms of parathyroid disorders.