Spectroscopic THz imaging of human skin can reveal hydration levels and yield insight into skin afflictions. However, in-vivo measurements necessitate careful studies of all variables that can affect the skin’s response. Here, we investigate how pressure affects the response of human skin during THz in-vivo investigations. Our findings show that increasing the pressure increases the refractive index. Therefore, inconsistent control of the pressure during THz in-vivo imaging will cause measurement inaccuracies and hinder valid diagnosis. Here we present how to monitor and control the pressure, our results can potentially be used for calibration of THz in vivo measurements.