Bioavailability of l-thyroxine and its metabolites after topical treatment with an emulsion containing 0.1% micronised l-thyroxine.

Aim: Aim of the study was to assess systemic effects of a cycle of treatment with a topical formulation of l-T4 and escin (Somatoline®) in healthy women based on changes in bioavailability of FT4, FT3, rT3, and TSH.

Methods: This study enrolled 20 healthy adult women with body mass index <30, not exposed to iodine-containing products. The study called for 28 consecutive days of treatment with Somatoline® followed by a 14-day follow-up period. Blood samples for FT4, FT3 and TSH levels were drawn at baseline, 5 and 24 hours after the first application and thereafter on days 14, 28 and 42. Levels of rT3 were measured during the first 24 hours postapplication.

Results: Subject mean age was 40.1±8.0 years and BMI from 19.1 to 29.8. Levels of FT4 always remained within normal range and did not change in a clinically relevant way from baseline (11±1.2 pg/dL), with maximum mean change from pretreatment values of 0.4 pg/mL (P=0.87). Likewise, FT3 and TSH levels did not change significantly from baseline (3±0.4 pg/dL and 1.8 ±0.9 µU/mL, respectively). Levels of rT3 behaved in a similar way, with modest changes from baseline (P=0.29). Local tolerability was defined "excellent" for 19 out of 20 women (95%) and "moderate" in one subject who experienced the onset of folliculitis, for which causal correlation with the treatment was considered "possible".

Conclusion: Used at the posology foreseen for the marketed formulation, Somatoline® does not affect plasma levels of FT4, FT3, rT3 and TSH, either in the short term or after 28 days.