Can a fourteen-year-old use?

You did not write what product you are referring to. If you mean the Endokrinol tagged with the question, it is intended for adults and should not be given to a fourteen-year-old on your own. The use of such a dietary supplement in children and adolescents should be supervised by a pediatrician, taking into account possible endocrine indications. Endokrinol contains, among other things, iodine, so it will not be suitable for all patients with thyroid disease. I enclose additional information that may be helpful - over-the-counter preparations can be purchased at GdziePoLek online: https://www.gdziepolek.pl/produkty/118806/endokrinol-tabletki/apteki