Metabolic studies were performed in a case of severe hypocalcemia and extensive osteoblastic metastases from prostatic adenocarcinoma in a 75-year-old man. The predominant abnormality appeared to be an increase in bone avidity for calcium, phosphorus, and magnesium, as evidenced by increased retention rates of infused calcium and magnesium and low urinary and stool values for calcium, phosphorus, and magnesium. Other aberrations included the presence of a relative hypoparathyroidism, peripheral resistance to parathyroid injection (United States Pharmacopeia), and subnormal plasma levels of magnesium and vitamin D metabolites. Prior to magnesium therapy there was no phosphaturic or cyclic adenosine monophosphate (cAMP) response to exogenous PTE, whereas after therapy there was normal cAMP but absent phosphaturic response. Hypomagnesemia together with relative parathyroid insufficiency could be responsible for the low 25-hydroxyvitamin D (25-[OH]-D) and 1,25-dihydroxyvitamin D (1,25-[OH]2 D) levels. The sporadic elevation in thyrocalcitonin levels was consistent with the possibility of ectopic production of this polypeptide. These study results strongly suggest that the hypocalcemia associated with osteoblastic metastases results from a number of interrelated abnormalities, chief of which is the great avidity of bone for calcium.
(Arch Intern Med 139:238-240, 1979)