A Single-Group, Open-Label Study on the Systemic Bioavailability, Safety, and Local Tolerability of a New L-Thyroxine/Escin Gel Formulation in Healthy Women

Objective: This study evaluated the systemic bioavailability of L-thyroxine (L-T4) in healthy women following repeated cutaneous application of a new gel formulation containing L-T4 and escin. Plasma concentrations of free triiodothyronine (FT3), reverse triiodothyronine (rT3), and thyroid-stimulating hormone (TSH) were also assessed, along with local and systemic tolerability. Methods: Thirty healthy women participated in a single-group, open-label trial. L-thyroxine gel was applied at 20 g/day for the first 2 days and 10 g/day for the following 26 days (equivalent to 20 mg/day and 10 mg/day of L-T4, respectively). Blood samples were collected at Baseline, 5 and 24 h after the first application, and on Days 14, 28 (End of Treatment, EOT), and 42 (End of Study, EOS). Tolerability and safety were monitored throughout. Results: Plasma FT4 concentrations remained stable throughout the study, with no clinically significant changes from Baseline (1.13 ± 0.15 ng/dL) to EOT (1.11 ± 0.13 ng/dL). FT3 and TSH levels also remained within physiological ranges, with only a transient, non-clinically relevant decrease observed 5 h after the first application. No changes in rT3 concentrations were detected at any time point. No serious adverse events were reported. Conclusions: This study confirms that repeated application of L-thyroxine/escin gel over 28 days (total exposure of 300 g) does not affect systemic thyroid hormone levels and is well tolerated in healthy women. These findings support the hypothesis that intact skin acts as an effective barrier to transdermal L-T4 absorption.