SUMMARY Disorders of calcium metabolism are not generally considered important either clinically or patbophysiologkally in essential hypertension. Recent reports, though, suggest that increased parathyroid gland function may be one of the more common endocrine disturbances associated with hypertension. We measured serum parathyroid hormone (PTH) concentrations as well as routine blood and arine chemistries in 34 hypertensives. Their mean PTH, 79.1 ± 3.1 filter Eq/ziUter, was rignlflcaatiy higher {p < 0.025) than the mean PTH, 66.9 ± 33, of an age-and sex-matched nonnotenshe control population. The mean serum calcium, 9.5 ± 0.1 mg%, was identical in the two populations. Compared to a second age-aad sex-matched normotensive population, the hypertensives demonstrated a significant (p < 0.005) relatiTe hypercalduria. For any lerd of urinary sodium, hypertenshes excreted more calcium. These preliminary data suggest that parathyroid gland function may be enhanced in essential hypertension. This increased gland actirity appears, in part, to be an appropriate, physiologic response to a previously unrecognized relative hypercalduria, or renal caldum leak, associated with essential hypertension. We condude that the increased prevalence of hypertension in subjects with byperparathyroidfam probably represents the final event In a continuum that begins with obligatory urinary caldum losses in hypertensives, but whose pathological presentation is hyperparathyroidism. The results of this pilot study indicate a need for derivative experiments directed at defining the importance of our preliminary findings In the patbogenesis of human and experimental hypertension. 1 "' Conversely, the prevalence of hyperparathyroidism may be two to eight times greater in hypertensive than in normotensive subjects. 4-• In addition, thiazide diuretics cause overt hypercalcemia in certain hyper-