OP31 – Perioperative Magnesium Levels in Total Thyroidectomy and Relationship to Hypocalcaemia

      Sheahan, Patrick1; Woods, Robbie1; Brophy, Catherine1; Geyer, Lina1; Collery, Charlotte1; Garrahy, Aoife2; Murphy, Matthew2 1 Department of Otolaryngology, South Infirmary Victoria University Hospital, Cork, Ireland 2 Department of Endocrinology, South Infirmary Victoria University Hospital, Cork, Ireland   Background: Hypocalcaemia is common after thyroidectomy. Magnesium plays an important role in calcium homeostasis via stimulation of parathyroid hormone (PTH) secretion and modulation of PTH receptor sensitivity.  Our aim was to evaluate trends in post-thyroidectomy magnesium levels and relationship with postoperative hypocalcaemia. Methods: Retrospective study of a prospectively collated database of 471 consecutive total thyroidectomies. Preoperative and postoperative day 1 and day 2 calcium and magnesium levels were collated.  Biochemical hypocalcaemia was defined as calcium <2mmol/L. Results: 295 patients had preoperative and day 1 postoperative magnesium levels available, of whom 232 also had day 2 postoperative magnesium levels available.  Among all patients there was a highly significant change in magnesium levels between preoperative values, and post-operative day one (0.87mmol/L to 0.80mmol/L, p<0.00001), and between post-operative day 1 and postoperative day 2 (0.80mmol/L to 0.78mmol/L, p=0.001). 95 patients developed biochemical hypocalcaemia. There was a strong correlation between post-operative magnesium and calcium levels, with change in magnesium levels significantly greater in patients developing hypocalcaemia (-0.1 mmol/L versus -0.05 mmol/L, p<0.00001). Between the first and second post-operative day, there was a significant drop in magnesium levels among patients developing biochemical hypocalcaemia (0.76 mmol/L to 0.73 mmol/L, p=0.0004), but not among patients remaining eucalcaemic (0.82 mmol/L to 0.81 mmol/L, p=0.30). Conclusions:  Downward trending of magnesium levels is common after total thyroidectomy.  This decrease is more marked in patients developing hypocalcaemia. It is possible that total thyroidectomy may induce hypomagnesaemia which can cause or exacerbate concomitant hypocalcaemia.


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