CD4 and focal dermal hypoplasia: The two-step method is thought to be an appropriate approach in FDH patients because almost all thyroxine-binding proteins are washed out before the labelled T4 analog penetrating into the incubation system, but indeed, unexpected high FT4 was also seen in FDH patients by using this assay method [30], especially in the Access system (Beckman Coulter) [28].