Subacute Thyroiditis in Denmark: A Nationwide Study of 1763 Cases

OBJECTIVE: Subacute thyroiditis (SAT) is a differential diagnosis in the clinical care of hypo- and hyperthyroid patients, but large-scale population data on the incidence, patient characteristics, and outcome are limited. Thus, the aim was to determine the incidence of SAT in the Danish population and to characterise patient demographics and the risk of permanent hypothyroidism. DESIGN: Retrospective cohort study based on Danish nationwide health registers. PATIENTS: Individuals diagnosed with SAT at Danish hospitals from 1995 to 2016, covering the years before and after the implementation of mandatory iodine fortification in Denmark as of July 2000. MEASUREMENTS: Annual incidence rates of SAT were calculated overall and by geographical region (East and West Denmark). The development of permanent hypothyroidism following SAT was evaluated based on redeemed prescriptions of levothyroxine (L-T4) within a 4-year follow-up period. RESULTS: A total of 1763 individuals were identified with a diagnosis of SAT among 1,747,168 individuals at risk. The overall nationwide incidence increased from 3.3/100,000 individuals (95% confidence interval (CI): 2.5-4.4) in 1995 to 6.2/100,000 individuals (95% CI: 5.1-7.6) in 2016, with no regional difference (p = 0.18). Follow-up showed that 459 of 1763 patients with SAT (26.0%) started treatment with L-T4, and in 224 of these patients (48.7%), the duration of L-T4 treatment exceeded 36 months. CONCLUSIONS: In a Danish nationwide cohort, the incidence of SAT increased during a 22-year period without geographical difference. Notably, the frequency of long-term L-T4 treatment was high, emphasising a focus on the post-diagnosis management of patients with SAT.