A COMPARISON OF NEPHROGENOUS CYCLIC AMP, TOTAL URINARY CYCLIC AMP AND THE RENAL TUBULAR MAXIMUM REABSORPTIVE CAPACITY FOR PHOSPHATE IN THE DIAGNOSIS OF PRIMARY HYPERPARATHYROIDISM

A determination was made of total urinary adenosine 3′-5′cyclic monophosphate (UcAMP), nephrogenous cyclic AMP (NcAMP) excretion and also of the renal tubular maximum reabsorptive capacity for phosphate T_mPO₄/GFR (all expressed as a function of the glomerular filtrate) in fourteen patients with primary hypercalcaemic hyperparathyroidism and twelve control normal subjects. The hyperparathyroid patients gave a mean excretion of UcAMP (7·0 ± 45·68 nmol/100 ml GF; mean ± SEM), NcAMP (6·19 ± 0·64 nmol/100 ml GF) which were significantly greater (P < 0·001) than those of normal controls, (2′45 ± 0·15nmol/100 ml GF and 1·25 ± 0·12nmol/100 ml GF) respectively. The difference between the patients and controls for the maximum renal tubular reabsorptive capacity for phosphate (T_mPO₄/GFR) (patients 0·55 ± 0·04, controls 1·05 ± 0·05 mmol/l GFR) was also highly significant (P<0·001). Statistical evaluation of the results obtained from the patients with primary hyperparathyroidism revealed that there was a positive correlation between the level of plasma calcium and immunoreactive parathyroid hormone (PTH) (r=+0·46), NcAMP(r=+0·337), UcAMP (r=+0·36), and an inverse correlation with the T_mPO₄/GFR (r=?0·62). There was also a positive correlation between plasma immunoreactive PTH and NcAMP(r=+0·31), and UcAMP(r=+0·35), and an inverse correlation with the T_mPO₄/GFR (r=?0–39). Successful removal of a single parathyroid adenoma in six patients was associated with a highly significant fall in the excretion of UcAMP, NcAMP, and a rise in the T_mPO₄/GFR (P<0·005). The combination of a low T_mPO₄/GFR and a high excretion of UcAMP or NcAMP in the presence of hypercalcaemia is highly suggestive of primary hyperparathyroidism in the absence of clinical evidence of malignant disease.