The pharmacokinetics of 2 doses (50 μG and 100 μG) levothyroxine treatment in athyreotic patients

Objective: Hypothyroid has still be a problem in Thailand. Lack of pharmacokinetics information of levothyroxine in athyreotic patients for modify dosing. Therefore, the aim of this study was to investigate the pharmacokinetic parameters of 2 doses of levothyroxine (50 μg and 100 μg) in such group of patients. Methods: The 24 athyreotic patients were recruited and randomly assigned to receive 50 μg or 100 μg of levothyroxine. The pharmacokinetic parameters (C_max, T_max, AUC_0-8, k_e, T_1/2 and V_d) of FT4 and TSH were calculated by the non-compartment model. The parameters of 2 doses were compared. Results: From the results found that, by monitoring FT4, there was no statistically significant difference (p>0.05) of almost all pharmacokinetic parameters between 2 doses except C_max which has significant greater in 100 mg of levothyroxine. While all pharmacokinetics parameters of TSH were not statistically significant difference. The use of non-compartment model seems appropriate for calculation of pharmacokinetic parameters of FT4 (R² = 0.81) while it has poor predictive capacity in terms of TSH (R² = 0.24). Conclusion: In conclusion, administration of 50 μg and 100 mg of levothyroxine to athyreotic patients have similar pharmacokinetic in term of FT4 (non-significant different in Tmax, AUC) and TSH. Further study of these parameters in more subjects is needed.