Dehydroepiandrosterone may influence thyroid function. Its metabolite, 7-oxo-dehydroepiandrosterone, a precursor of immunomodulatory 7-hydroxylated metabolites and thermogenic agent, belongs to candidates of steroid replacement therapy. The question was addressed whether its application does influence laboratory parameters of thyroid function. 7-Oxo-dehydroepiandrosterone in the form of emulgel, 25 mg/day, was applied transdermally to 21 healthy men for 8 consecutive days. Morning blood was collected before the treatment (Day 0, Stage 1), during treatment (Day 5, Stage 2), on the first day after the last administration (Day 9, Stage 3), one week (Day 16, Stage 4), and 9 weeks (Day 72, Stage 5) after treatment termination. The levels of thyrotropin, free thyroxine and triiodothyronine, dehydroepiandrosterone, its sulfate and its 7-hydroxyepimers were measured. The changes were evaluated by analysis of variance and correlation analysis. During treatment a significant rise of 7beta-hydroxy-dehydroepiandrosterone was observed, which persisted 1 week after treatment termination. No changes were observed in dehydroepiandrosterone and its sulfate. Though a slight but significant rise of TSH and of both thyroid hormones occurred during treatment, its levels soon returned to the basal values. It was concluded that treatment of 7-oxo-dehydroepiandrosterone affects the thyroid parameters only temporarily and that it provides a considerable persistent amount of 7beta-hydroxy-dehydroepiandrosterone.