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Giornale Italiano di Dermatologia e Venereologia 2003 August;138(4):333-40


language: Italian

Study on the systemic bioavailability of L-T4 after repeated cutaneous application of a commercially available topical drug

Righini V. 1, Renzetti D. 2, Bevilacqua M. 1

1 UO di Endocrinologia Azienda Ospedaliera L. Sacco, Milano 2 Pulsar sas, Studi Clinici e Biometrici, Monza (Milano)


Aim. The aim of this work is to ascertain if the repeated percutaneous application of a known drug in the form of emulsion, containing 10 mg of l-thyroxin/sachet, in comparison with a new emulsion having the same content in the active ingredient, causes the systemic availability of the exogenous hormone, modifying the serum levels of T3, T4, FT3, FT4, TSH and TBG.
Methods. The study was carried out in a double-blind cross-over design, by allotting at random the 2 sequences of treatment to 20 healthy female volunteers meeting the selection criteria. Each subject applied onto both thighs each 1 of the 2 formulations once a day, for 3 weeks. Moreover, the routine biohumoral tests, the vital indices, the 24-hour diuresis and the ioduria were carried out before the beginning, during and at the end of each 1 of the 2 cycles of treatment. The effects of the 2 formulations were assessed by two-tail Anova. H0 was rejected at the probability p≤ 0.05.
Results. The 2 formulations were found to be bioequivalent. Actually, no biological-clinical and statistical (p>0.05) variations of the baseline levels of the tested variables were found, in particular of the hormonal ones and of TBG, both in acute—after the first 2 applications—and at the end of the 21 days of each cycle of treatment. Moreover, both formulations were manageable and well tolerated also topically, save a mild erythematous reaction in the site of the application which appeared in 1 subject the 20th day of the 2nd treatment. However, it did not prompt the treatment discontinuation and resolved spontaneously after few days.
Conclusion. This trial demonstrated that the cutaneous application for 21 consecutive days of l-thyroxin causes no clinical and statistical variations (P>0.05) of the serum levels of T4. This supports the hypothesis that T4 concentrates in the skin or dermis, from which it is removed with the cutaneous turnover or by local metabolic deactivation.

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