Objective: A decrease over time in thyroid stimulating hormone (TSH) leve ls when initiating levothyroxine (L-T4) therapy for hypothyroidism has been reported, where treatment most ofte n is initiated with TSH levels below 10 mIU/L. The primary objective of this study was to investigate whether this lower TSH threshold resulted in an increased number of overtreated patients. Design and method: Retrospective cohort study comprising inhabitants in Copenhage n had TSH measurements requested by general practitioners which led to a new prescript ion of L-T4 between 2001 and 2012. Over- and under-treatment were defined as TSH <0.1 mIU/L or above 10 mIU/mL, respectively, in three consecutiv e measurements. Data were analyzed by Aalen-Johansen estimators and Cox proport ional hazards models. Results: In total, 14 533 initiations of L-T4 were included in the stud y. The cumulative risk of being over- or undertreated was 4.7 and 7.4% after 10 years. The hazard of ove rtreatment was higher among women, younger adults, and with lower initial TSH levels. The hazard of overtr eatment decreased over the time period from 2001 to 2012. Among overtreated individuals, the chance of returning to a normal TSH was about 55% after 10 years. In 18% of the cases, L-T4 therapy was initiated on TSH levels less than 5 mIU/L. Conclusion: Although a still decreasing threshold for initiating L-T4 ther apy is known, the risk of overtreatment (and undertreatment) was low and lessened in the period 2001-2012 am ong Danish primary care patients. Nevertheless, as many as 18% were started on L-T4 with normal TSH levels 2021 European Society of Endocrinology Printed in Great Britain.