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Hypoparathyroidism is the most common complication after thyroid surgery and associated with short- and long-term consequences. The lack of a consensus on the definition of hypoparathyroidism has led to a broad range in the rates reported in the literature. The overall aim of this thesis was to study different aspects of hypoparathyroidism, in terms rates, risks and long-term impact. Paper I is a case control study, investigating prophylactic, preoperative treatment with active vitamin D and early hypocalcemia after total thyroidectomy. The study showed that patients with preoperative treatment had a lower risk of early hypocalcemia and a reduced length of stay in hospital, compared to patients without treatment. No adverse outcomes were found. Paper II is a population-based retrospective cohort study on the rate and risks for permanent hypoparathyroidism after total thyroidectomy for benign thyroid disease. Data was retrieved from The Swedish National Patient Register, The Swedish Quality Register for Thyroid, Parathyroid and Adrenal Surgery and The Swedish Prescribed Drug Registry. Permanent hypoparathyroidism was defined as dispensation of calcium and/or active vitamin D >12 months after surgery. Among 7852 patients, 12.5% developed permanent hypoparathyroidism. Surgery at low volume centers, parathyroid autotransplantation, female gender and high age were independent risk factors. In Paper III the aim was to validate the high rate of permanent hypoparathyroidism found in Paper II. A regional cohort was extrapolated from the national cohort. A retrospective chart review, of 1636 patients, was performed. Using a strict definition, 6.2 % were found to have definitive permanent hypoparathyroidism. Additionally, 2.5 % were found to have possible permanent hypoparathyroidism. Of these, at least 1.7 % might have been overtreated due to lacking attempts to unwind the treatment. The study also proposed that the rate of low early PTH in a cohort might be useful to predict the rate of permanent hypoparathyroidism. Paper IV investigated health related quality of life (HRQoL) in patients with and without permanent hypoparathyroidism using the same cohort as in Paper III and SF-36 v.2. No impact of definitive hypoparathyroidism on HRQoL could be found. In conclusion, the use of preoperative active vitamin D may be useful as a tool to lower the risk of early hypocalcemia. The risk of permanent hypoparathyroidism after total thyroidectomy is high and there is a need for improved follow up. Permanent hypoparathyroidism may not have a negative effect on HRQoL in most patients.