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

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.

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.

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".

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.