Secondary hyperparathyroidism of morbid obesity regresses during weight reduction

In order to test the relation between obesity and the secondary hyperparthyroidism found in markedly overweight subjects, 24 morbidly obese patients were studied before and after a weight loss of 35.9 kg obtained by a nutritionally adequate, intermittent very-low-calorie diet. Overweight was reduced from 98 ± 34% to 44 ± 19%. Serum total calcium did not change, but serum ionized calcium (Ca²⁺) increased from 1.22 ± 0.04 mmol/L to 1.25 ± 0.04 mmol/L (P < .001). A corresponding fall was observed in serum parathyroid hormone (s-PTH), which decreased from 47.2 ± 21.7 pmol/L to 35.2 ± 19.4 pmol/L (P = .01). The change of s-PTH was positively associated with the reduction of body weight (r = .50, P < .05) and with the reduction of overweight (r = .55, P < .01). Regarding calcium binding substances, serum albumin remained low. The initially lowered serum phosphate and bicarbonate both rose (P < .001). Plasma lactate and plasma free fatty acids (FFAs) decreased (P < .001). The study supports our hypothesis that the changed profile of calcium complexing anions in obesity interferes with the tubular reabsorption of calcium, which in turn lowers serum Ca²⁺, thus promoting hyperparathyroidism. Along with weight loss, concentrations of calcium complexing anions returns towards normal values and the secondary hyperparathyroidism regresses.