Context. Several enlarged parathyroid glands could be found during thyroid surgery in normocalcemic patients without evidence of primary or secondary hyperparathyroidism, indicating multiglandular parathyroid gland disease (MGD). Objective. Clinical role of various levels of serum ionized calcium (Ca 2+) in patients diagnosed with incidental MGD during thyroid surgery remains controversial. The aim of the study was to evaluate the features of PHPT and the clinical role of serum Ca 2+ in normocalcemic patients diagnosed with incidental MGD. Study design. A prospective study of patients with normal preoperative Ca 2+ to be operated on for thyroid diseases in 2010-2013 and diagnosed with MGD during thyroid surgery. Methods. An analysis of clinical data from 3,561 patients to be surgically treated for thyroid diseases revealed 219 (6%) patients with MGD and normal serum Ca 2+. Further data analyses showed patients with MGD and high normal (≥1.25-1.3 mmol/L) serum Ca 2+ (n = 89) and with moderate-low (1.0-1.24 mmol/L) serum Ca 2+ (n = 130). Results. Primary hyperparathyroidism was diagnosed intra-and post-operatively in 48 (54%) patients with high-normal serum Ca 2+ and in 2 (2%) patients with moderate-low serum Ca 2+ (p<0.0001). Parathyroid hormone, serum Ca 2+ as well as urine calcium excretion were elevated in 2 (2%) patients with moderate-low serum Ca 2+ and in 18 (20%) patients with high-normal Ca 2+ at follow-up (p<0.0001). Conclusion. Serum Ca 2+ level within the normal range, but higher than 1.25 mmol/L (high-normal) is associated with primary hyperparathyroidism, which should be considered in patients with visually diagnosed MGD, but without clinical symptoms of hyperparathyroidism.

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