Increased parathyroid hormone as a consequence of changed complex binding of plasma calcium in morbid obesity

To evaluate whether changed plasma calcium binding might lead to a secondary increase of parathyroid hormone in morbid obesity, fasting measurements of serum ionized, ultrafiltrable and total calcium, calcium binding substances, and parathyroid hormone were undertaken in age- and sex-matched groups of obese (n = 44) and normal weight subjects (n = 52). The 24-hour urinary calcium excretion and clearance of creatine were also measured. Calcium binding to proteins was changed. Serum total proteins and protein-bound calcium did not differ, but serum albumin was decreased in obesity. Consequently, obese subjects did not reveal the normal dependency of protein-bound calcium upon albumin. Calcium binding to other substances was also changed. Serum phosphate and bicarbonate were decreased, while the concentrations of citrate, lactate, acetoacetate, 3-hydroxybutyrate, free fatty acids, and urate were all increased, leaving the total concentration of plasma complex-bound calcium unchanged. Nevertheless, these reciprocal changes increase the concentrations of less readily reabsorbable anions in the renal ultrafiltrate. The changed pattern of calcium binding in serum of the obese subjects may serve to explain our findings of increased urinary calcium excretion, lowering of serum ionized calcium and increased parathyroid hormone levels, changes being significantly correlated with degree of overweight.