Preparation of Levothyroxine Transdermal Gels and Preliminary Pharmacokinetic Study in Hypothyroidism Rat Model

Background: Hypothyroidism is a common disorder due to inadequate thyroid hormone secretion. In patients with hypothyroidism, levothyroxine (LT4) is the treatment of choice, and tablets are the most common dosage form. However, the main limitation of tablet LT4 is malabsorption. Objective: This study intends to develop a new dosage form of percutaneous drug delivery for levothyroxine. Absorption of levothyroxine sodium through the application of gel formulation was studied using a hypothyroidism rat model. Methods: A formulation of levothyroxine sodium gel was developed and selected. In-vitro transdermal experiments were performed using the vertical Franz diffusion pool method, and gel formulation was used for animal research (hypothyroidism rats model). Total 30 rats were randomly divided into 6 groups, and one was the normal control group. The other 5 groups were prepared as hypothyroidism models. After applying different doses of gel preparation to the rat model, we measured serum total thyroxine (TT4), free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) using fluorescence determination of luminescence immunoassay. Results: The optimum formulation of levothyroxine gels comprised 20% polyvinyl alcohol (PVA), 5% glycerol, 2% azone, and 6% oleic acid. The application of levothyroxine sodium gel resulted in quick and smooth action so that the predicted level of the normal control group could be reached within 2 weeks, and it lasted steadily for 8 weeks. Conclusion: This research study successfully developed and tested an optimal formulation of levothyroxine gel with therapeutic benefit on hypothyroidism in rats.