Seasonal variation in calcium treatment after thyroidectomy as surrogate for post-operative hypocalcemia: a retrospective register-based national cohort study

BACKGROUND: Hypocalcemia is one of the most common complications of thyroidectomy, and vitamin D deficiency has been found to be an independent risk factor. Sweden is located north of the 55th latitude, resulting in a significant seasonal variation in sun exposure, thereby large variation in the naturally occurring levels of vitamin D. This study aimed to determine if there is a correlation between season of surgery and post-thyroidectomy hypocalcemia. METHODS: We conducted a retrospective register-based observation study on patients who had undergone total thyroidectomy during 2008–2015. In total, 7125 patients operated in Swedish facilities were identified via the Scandinavian Quality Register for Thyroid, Parathyroid, and Adrenal Surgery (SQRTPA). Patients operated during February–April were included in the dark group and patients operated during August–October were included in the bright group. Further stratification was made on the indication for surgery. The primary outcome was post-operative calcium treatment due to hypocalcemia, defined as having received calcium orally or intravenously before discharge. RESULTS: The risk of receiving post-operative calcium treatment was significantly lower in the bright group (29.7%) compared to the dark group (35.1%), with a relative risk of 0.846 (P < 0.001). This correlation held true if the indication for surgery was goiter or thyrotoxicosis. For malignancy, there was no significant difference between the groups. CONCLUSION: In this cohort, total thyroidectomy performed during August–October was associated with a lower rate of calcium treatment given post-operatively when compared to total thyroidectomy performed during February–April. This would indicate a decreased risk of post-operative hypocalcemia if surgery was carried out after the brighter season.