Relationships of serum 25-hydroxyvitamin D, ionized calcium and parathyroid hormone after obesity surgery

Objective: The high prevalence of secondary hyperparathyroidism (SHPT) after obesity surgery is a concern for long-term bone health. Limited knowledge exists about optimal vitamin D and suppression of parathyroid hormone (PTH) after these procedures. The aim of this study was to investigate the prevalence of SHPT and its relation to vitamin D status.

Design: A cross-sectional study at Oslo University Hospital, Norway.

Patients: A total of 502 consecutive patients, age 22-64 years, attending 2-year follow-up after Roux-en-Y gastric bypass.

Measurements: A serum intact PTH >7.0 pmol/L in the absence of elevated serum ionized calcium (iCa) was considered as SHPT. Vitamin D status was defined by serum concentrations of 25-hydroxyvitamin D (S-25(OH)D).

Results: Altogether, 171 patients (34%) had SHPT. The prevalence of SHPT varied across the range of S-25(OH)D (P < 0.001), being highest (71%) with S-25(OH)D < 25 nmol/L. Compared with S-25(OH)D < 50 nmol/L, the prevalence of SHPT was lower with S-25(OH)D ≥ 50 nmol/L (29.0%; RR = 0.64 (95%-CI:0.50-0.81)) and S-25(OH)D ≥ 75 nmol/L (27.7%; RR = 0.61 (95%-CI:0.44-0.84)). S-25(OH)D ≥ 100 nmol/L was associated with the lowest PTH and the lowest prevalence of SHPT (16.0%; RR = 0.35 (95%-CI:0.14-0.88) compared with S-25(OH)D < 50 nmol/L) and the most normal calcium distribution. These associations were most pronounced with iCa in the lower range. A synergistic association was found for S-25(OH)D and iCa on SHPT.

Conclusions: Vitamin D deficient patients had the highest prevalence of SHPT 2 years after gastric bypass. PTH and the prevalence of SHPT were notably lower with S-25(OH)D ≥ 100 nmol/L, compared with lower target levels.