Evidence check: FT3 in the context of Haut

FT3 (free triiodothyronine) is the active thyroid hormone and influences skin physiology via nuclear receptors in keratinocytes and fibroblasts. However, the evidence linking FT3 levels directly to common skin conditions (dryness, eczema, acne) is mostly mechanistic or based on small observational studies. Clinically relevant skin changes are typically seen only in overt thyroid dysfunction (hyper-/hypothyroidism). A single FT3 measurement without TSH and fT4 is difficult to interpret. Consumer finger-prick tests can provide a screening hint but lack diagnostic accuracy and have preanalytical variability. For skin concerns, a dermatologist should be the first point of contact; thyroid workup requires medical supervision. Evidence: mechanistic/unclear. Caveat: do not self-medicate with thyroid hormones.