The effects of calcium homeostasis and thyroidectomy: Postoperative hypocalcaemia complicating thyroid surgery

Hypocalcaemia is the most common complication of total or completion thyroidectomies and the reported incidence of this potentially fatal condition varies widely in the literature. Hypocalcaemia develops between two to five days after thyroid surgery. This raises three main issues, namely increased patient hospital stay, the risk of patient discharge before the condition develops and an assessment of its risk influences the timing of patient discharge. For these reasons, a model for predicting which patients require calcium post-surgery is required. The proposed benefits of such a model include a reduced length of stay, reduced risk of re-admission and the opportunity to commence calcium supplements before the onset of symptoms of hypocalcaemia. This study defines a patient requiring postoperative calcium supplements as either having an adjusted calcium measurement below 2.0 mmol/L at any time up to one year following surgery or being administered calcium supplements after presenting clinical signs (Trousseau’s sign or Chvostek’s sign) or symptoms (such as perioral numbness) of hypocalcaemia. A retrospective study of patient records and pathology results of patients having undergone either total or completion thyroidectomy at either The Geelong Hospital or St. John of God Hospital during the years 2001 – 2008 was undertaken. The statistical association between the patient outcome of requiring postoperative calcium supplements and a number of commonly evaluated categorical risk factors (including age, gender and procedure) were evaluated. Novel categorical risk factors (such as patient smoker status and medications at admission) were also investigated. Receiver-Operating Characteristic curve models were generated to evaluate the ability of biochemical markers to predict which patients require postoperative calcium supplements. Models based on calcium, parathormone, phosphate, magnesium, thyroxine, triiodothyronine and thyroid stimulating hormone measurements were investigated. One hundred and eighty patients undergoing total or completion thyroidectomy at The Geelong Hospital or St. John of God Hospital during the years 2001 – 2008 are included in the current study. The mean age at surgery was 51.3 years and the female-to-male ratio was 5.2:1. There were 145 total thyroidectomies performed and 35 completion thyroidectomies. The median length of hospital stay was 3.0 days. Forty-two patients developed an adjusted calcium measurement below 2.0 mmol/L within one week of surgery and fourteen patients showed signs of hypocalcaemia and were commenced on calcium supplement therapy. Risk categories for which there were no statistically significant associations with the patient outcome include: gender, age at surgery, procedure, number of parathyroid glands excised, duration of surgery, patient smoker status, blood group, the use of medications and the presence of co-morbidities concurrent with surgery. This study identifies a patient management model based on the early prediction of which patients require postoperative calcium supplements. A single PTH measurement at 6 - 8 hours post-surgery below 1.1 ρmol/L identified all patients requiring postoperative calcium supplements (100 % sensitivity) and ruled out 85.7 % of patients not requiring postoperative calcium supplements. Implementing this model would facilitate the safe discharge on the day of surgery of patients undergoing completion thyroidectomy before 11:00AM. All other patients could be discharged on the first postoperative day, with a PTH measurement below 1.1 ρmol/L at 18 – 24 hours identifying with 91 % sensitivity which patients require postoperative calcium supplements. A second finding of the study lends support to Hungry Bone Syndrome as a mechanism by which hypocalcaemia complicates thyroid surgery. Whilst no statistically significant association could be identified, it was found that patients suffering from Graves’ disease required postoperative calcium supplements at a rate that was elevated as compared to all other thyroid pathologies. This finding is also supported by statistical models generated from thyroid hormone measurements for the detection of which patients require postoperative calcium supplements.