A COMPARISON OF PARATHYROID-GLAND FUNCTION IN HEMODIALYSIS-PATIENTS WITH DIFFERENT FORMS OF RENAL OSTEODYSTROPHY

Parathyroid function was studied in three different histological forms of renal osteodystrophy: osteitis fibrosa (OF), low-turnover aluminium-associated bone disease (LTAABD), and aplastic bone disease without aluminium (ABD). Parathyroid function was determined by the evaluation of the sigmoidal parathyroid hormone-(PTH)-calcium curve, which was obtained by the performance of a reduced calcium and an increased calcium haemodialysis. Parameters of the sigmoidal PTH-calcium curve evaluated included maximally stimulated (PTH(Max)) and inhibited (PTH(Min)) PTH, the set point of calcium for PTH (ICA50), defined as the ionised calcium concentration at which PTH(Max) was reduced by 50%, the ratio of basal PTH to maximally stimulated PTH (PTH(Base):PTH(Max)), the ionised calcium concentration at which basal (ICA(Base)), maximally stimulated (ICA(Max)), and maximally inhibited (ICA(Min)) PTH values were observed, and the slope of the PTH-calcium curve. Both PTH(Max) and PTH(Min) were greater in OF than the other two groups (P < 0.02). The ratio of basal to maximally stimulated PTH was greater (P < 0.02) in OF (61 +/- 7%) than LTAABD (33 +/- 5%) and ABD (36 +/- 7%). The ICA50 and the ICA(Max) were greater (P < 0.03) in OF than the other two groups; however, no differences were observed in the ICA(Base) and ICA(Min). The slope of the PTH-calcium curve (% maximal PTH), which should indicate the sensitivity of parathyroid cells, was greater in OF than LTAABD (P < 0.04). No differences in the PTH-calcium curve were observed between LTAABD and ABD. An additional finding was a significant correlation between PTH(Min) and PTH(Max) (r = 0.93), and PTH(Base) and PTH(Max) (r = 0.87), overall, and within each of the three individual groups. In conclusion (1) the linear correlation between PTH(Min) and PTH(Max) suggests that these parameters may reflect parathyroid gland mass; (2) parathyroid function was similar in LTAABD and ABD; and (3) in osteitis fibrosa: (a) the greater PTH(Base):PTH(Max) ratio indicates increased basal stimulation of PTH secretion; (b) the greater ICA50 and ICA(Max) indicates a shift of the PTH-calcium curve to the right; and (c) the steeper slope of the PTH-calcium curve (OF versus LTAABD) indicates that the sensitivity of parathyroid cells was enhanced.