16: Further evidence for a feed—back loop between parathyroid hormone (PTH) and vitamin D (vit D) metabolites: acute decrease of serum immunoreactive PTH (iPTH) in normocalcemic vitaminD deficient ricket (vit D. D. R.) after vitamin D₂ load

Abstract

The secondary hyperparathyroidism (HPT) present in normocalcemic vit D.D.R. is not clearly understood. Recent studies indicating that a feed-back loop may be present between PTH and 1,25 dihydroxycholeoalciferol, we have examined the acute effect of a vit D load on serum iPTH levels in 5 infants (age 5 to 11 months) with normocalcemic vit D.D.R. They all presented a clinical history compatible with vit D deficiency and marked typical bone lesions of rickets. Serum Ca ranged from 8,9 to 9,6 mg/100ml, serum P from 3,8 to 5,5 mg/100ml and serum alkaline phosphatases(A.Pase) from 84 to 225 iu(N<50). Elevated serum iPTH levels were present: 280 to 670 μlEq/ml(N<100). A unique oral vit D₂ load (600 000 u) was given. In all infants a marked decrease in serum iPTH was observed 24 and 48 h later (serum iPTH at 48 h : 115 to 360 μlEq/ml -ΔiPTH : 140 to 450 μlEq/ml) with no consistent changes in serum Ca, P and A.Pase. Normal levels of serum iPTH were found on a control 10 to 18 days later, though serum A. Pase remained elevated and healing of bone lesions was incomplete. These results suggest that the HPTof normocalcemic vit D.D.R. is, at least in part, directly secondary to the lack of active metabolites of vit D, probably 1,25 dihydroxy vit D; they are also consistent with the concept of a feed-back loop between PTH and vitamin D metabolites.

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