Backgorund: Levothyroxine, a first-line therapy for hypothyroidism, is a drug with a narrow therapeutic index, used in a very small amount (μg). These features make LT4 particularly sensitive to various factors interfering with its absorption, e.g. food and drugs. These interferences are usually clinically relevant. Chronic use of LT4 therapy and the recommended timing of LT4 administration, half an hour before breakfast (according to the SmPC), which is not suitable for many patients, increase the risk of reduced adherence. Aim of study: The aim of this study was to investigate effects of different timing of LT4 administration (A – half an hour before breakfast; B – an hour before the main meal of the day; C – at bedtime (minimally 2 hours after dinner)) on the parameters of thyroid function status and lipid profile of the patients. Methods and patients: Study included 84 patients with the diagnosis of primary hypothyroidism and using a stable dose of LT4. Patients were randomized into three different groups (A, B or C) and passed all different timings of LT4 administration (each lasted for at least 8 weeks), in a crossover fashion. Serum concentrations of parameters of thyroid function (thyroid-stimulation hormone (TSH), free thyroxin (fT4) free triiodothyronine (fT3) and lipid profile (triglycerides, high density lipoprotein- (HDL), low density lipoprotein- (LDL), and total cholesterol) were measured at the beginning of every timing regime and at the end of the study. Results: The analysed results did not showed any statistically significant differences in parameters of thyroid function (TSH, fT4, or fT3) or the lipid profile (triglycerides, HDL-, LDL-, and total cholesterol) when compared between three different timing regimens of LT4 administration. Considering stability of LT4 supplemental therapy, there was no difference among the calculated coefficients of variation of the thyroid function and lipid profile parameters nor the difference in percentage of patients whose values of the examined parameters were within the corresponding reference intervals after different timing regimes of LT4 administration. Conclusion: Since there was no difference in effectiveness in regulating patients’ thyroid function and lipid profile between different timings of LT4 administration, this study provides additional options regarding the personalization of pharmacotherapy and achievement of increased efficacy of the treatment, especially for the patients who did not reached therapeutic goals with standard way of administration.