Context: The rate of thyroid cancer is increasing in France, as well as concerns about overdiagnosis and treatment. Objectives: To examine the care pathway of patients who undergo thyroid surgery in France and detect potential pitfalls. Design: A large observational study based on medical reimbursements, 2009–2011. Setting: Data from the Sniiram (National Health Insurance Information System). Patients: Patients with thyroid surgery in 2010, classified into 4 groups: thyroid cancer, benign nodule, goitre or multiple nodules, other (hyperthyroidism, head–neck cancer). Main outcome: measures Medical investigations during, prior and after thyroidectomy. Results: A total of 35 367 patients underwent surgery (mean age 51 years, 80% women): 17% had a reported diagnosis of thyroid cancer, 20% benign nodule, 38% goitre or multiple nodules and 25% another diagnosis. The ratio of thyroidectomies with cancer over thyroidectomies with benign nodule was 0.8 and varied across regions. In the year preceding surgery, 82% of patients had an investigation by thyroid ultrasonography, 21% thyroid scintigraphy, 34% fine-needle aspiration cytology, 40% serum calcitonin assay and 54% serum calcium assay. In the following year, all patients with total thyroidectomy and 44% of patients with partial thyroidectomy and a diagnosis of benign nodule were taking thyroid hormone therapy. 100 patients had been reoperated for a compressive haematoma and 63 died during the first month, half of whom had been operated for cancer. Mean rates of recurrent laryngeal nerve injury and hypocalcaemia (requiring blood tests plus treatments within 4–12 months) were estimated at 1.5% and 3.4%, respectively, and were higher in the cancer group (2.3% and 5.7%). Conclusions: This almost nationwide study demonstrates the suboptimal management of patients prior to thyroidectomy in France. It suggests overdiagnosis and potential harms to patients, and calls for a review of the relevance of thyroidectomy, particularly with regard to microcancers.