Hypercalcemia of seven years' duration after kidney transplantation

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

Stones in renal transplants

In a 50-year-old man (case 1) and a 17-year-old girl (case 2) stones developed in a functioning renal transplant. The respective intervals from transplantation to appearance of stone were seven years and six months. The serum calcium was elevated to approximately 3.00 mmol/l in case 1 and 2.65 mmol/l in case 2. The concentration of parathyroid hormone in serum was increased in case 1. Subtotal parathyroidectomy was performed in both patients, with removal of 2550 mg (case 1) and 150-160 mg (case 2) parathyroid tissue. In case 2 slight hypercalcaemia reappeared two months later. When stones in a transplanted kidney are associated with only slight hypercalcaemia and there is no deterioration of the renal function, conservative measures to reduce the serum calcium should be tried before subtotal parathyroidectomy is undertaken.

Calculi in renal transplants

Three patients who underwent live donor renal transplantation subsequently developed calculi in their allografts. Hypercalcaemia and secondary hyperparathyroidism were present in 2 cases and these were treated by subtotal parathyroidectomy. Urinary stagnation and infection were contributory factors in the third case and reimplantation of the ureter was necessary. In all patients no further calculi have developed following treatment and allograft function remains satisfactory.