The Effect of Hypomagnesemia on Refractory Hypocalcemia after Total Thyroidectomy: A Single-Center Prospective Cohort Study

Background: Hypomagnesemia is known to impede hypocalcemia correction. This prospective observational study aimed to evaluate the impact of serum magnesium levels on the development of refractory hypocalcemia, which remains a concerning problem after total thyroidectomy (TT). Subjects and Methods: Consecutive subjects (n = 312; mean age = 38.4 [range: 13–83] years; M:F = 62:250) undergoing TT for benign or malignant thyroid diseases were evaluated for serum corrected-calcium (8.4–10.4 mg/dL), magnesium (1.7–2.4 mg/dL), intact parathormone (iPTH), and 25-hydroxycholecalciferol (25OHD) levels preoperatively, at 48-h and 6-month post-TT. Results: Postoperatively, 98 subjects (31.4%) exhibited transient hypocalcemia, 96 (30.8%) had hypomagnesemia, and 52 (16.7%) had refractory hypocalcemia. Preoperatively, 38 subjects (12.2%) had asymptomatic hypocalcemia and 77 (24.7%) had hypomagnesemia. In multivariate logistic regression analysis, independent risk factors of transient hypocalcemia were hyperthyroidism (odd's ratio [OR]: 5.6), 48-h iPTH (OR: 3.2), 48-h magnesium (OR: 2.7), preoperative 25OHD (OR: 0.96), and preoperative calcium (OR: 0.5; each P < 0.01). In receiver-operating characteristic analysis, percent calcium decline and 48-h magnesium reliably predicted transient hypocalcemia with a threshold of 10.5% and 1.9 mg/dL, respectively. Area under curve, sensitivity, and specificity were 0.822, 82.7%, and 72.9%; and 0.649 (each P < 0.001), 68.4%, and 63.1%, respectively. Conclusion: Serum magnesium below 1.9 mg/dL had 2.7 times higher odds of developing transient hypocalcemia post-TT. Hypomagnesemia and percent calcium decline >10.5% within 48-h post-TT are associated with refractory hypocalcemia, which necessitates correction of both the deficiencies for prompt resolution of symptoms.