An underlay, overlay, or wrap(s) through which temperature-regulated fluid is circulated, as part of an external circulating-fluid core temperature regulation system, intended to elevate and/or lower a patient’s core body temperature, typically in the operating room (OR), intensive care unit (ICU), or a recovery unit. The underlay/overlay is available in a variety of lengths, widths, and thicknesses to accommodate body size and application (e.g., adult/paediatric). It is intended to be used by a healthcare professional in a clinical setting. This is a reusable device.