In the treatment of hypothyroidism, thyroid-stimulating hormone (TSH) and thyroid hormones are measured and the levothyroxine dose is adjusted in order to achieve a euthyroid state. The initial dose is calculated according to weight, but other factors can influence the dose required to stabilize TSH.Objective: To estimate the percentage of patients who achieve good control of TSH over a 4-year follow-up period; to assess the factors associated with TSH instability.Design: Retrospective population-based cohort study of patients treated for hypothyroidism in a health region in Catalonia, Spain (2010Spain ( -2014. The data were obtained from the pharmacy drug dispensation register included in the computerized medical records of the regional health system. Setting: Lleida health region (Catalonia, Spain).Participants: All patients who had an initial normal TSH value (0.27-5.0 mU/L) or had received levothyroxine were selected. Main measurements:The variables analyzed were: TSH values during follow-up, estimated time with normal TSH, age, sex, body mass index, and compliance with treatment.Results: Of the 2,630 patients with hypothyroidism, 90.3% were women. Mean age was 59.6 years and the treatment compliance rate 84.9%. Patients taking a medium dose of levothyroxine (75-125 mcg) and older patients had higher TSH stability and maintained normal TSH levels for longer (p<0.05). Conclusions:A fifth of patients with hypothyroidism had inadequate thyroid hormone replacement in the first year. Factors associated with TSH stability were levothyroxine dose in the medium range and older age.