Objective: A decrease over time in thyroid stimulating hormone (TSH) levels when initiating levothyroxine (L-T4) therapy for hypothyroidism has been reported, where treatment most often 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 Copenhagen who had TSH measurements requested by general practitioners which led to a new prescription of L-T4 between 2001 and 2012. Over- and undertreatment were defined as TSH < 0.1 mIU/L or above 10 mIU/mL, respectively, in three consecutive measurements. Data were analysed by Aalen-Johansen estimators and Cox proportional hazards models. Results: In total, 14,533 initiations of L-T4 in the study. The cumulative risk of being over- or undertreated, was 4.7% and 7.4% after 10 years. The hazard of overtreatment was higher among women, younger adults and with lower initial TSH levels. The hazard of overtreatment 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 therapy is known, the risk of overtreatment (and undertreatment) was low and lessened in the period 2001 – 2012 among Danish primary care patients. Nevertheless, as many as 18% were started on L-T4 with normal TSH levels.

Introduction High compared with low educational level increases the odds of starting levothyroxine (L-T4) with a normal TSH – the mechanism is most likely patient request. The use of liothyronine (L-T3) and desiccated thyroid extract (DTE) is also speculated to be initiated at patients’ request. Therefore, the primary aim of this study was to evaluate if educational level influences treatment with L-T3 and DTE. Material and methods In this register-based cross-sectional study we included all Danish citizens ≥30 years with redeemed prescription of L-T4, L-T3, or DTE during 2017- 2020. We defined educational levels as short, medium, and long (<10 years, 10-12 years, and above 12 years, respectively). The association between educational level and treatment with LT3 or DTE vs. only LT4 was analyzed in logistic regression models adjusted for age and sex. Results We included 154,360 individuals using thyroid medication of whom 3,829 were treated with L-T3 (2.48%) and 430 with DTE (0.28%). The usage was highest among women (3.15%) and the age group 40–49 (5.6%). Longer education compared with short increased the odds of being treated with DTE or L-T3 (medium education OR 1.61 (95% CI 1.50–1.8) and long education OR 1.95 (95% CI 1.79–2.13)). Test for trend OR 1.37 (95% CI 1.31–1.42). Adjustment for other covariates did not affect the results substantially. Conclusion Persons with a longer compared to a shorter education are more often treated with either DTE or L-T3, and the usage of these drugs is limited to less than 3% of thyroid hormone users.

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