Implantable Medical Devices (IMDs) are small devices designed to be safely implanted inside the human body, playing a crucial role in advancing biomedical telemetry. This article proposes the design of a miniaturized Planar Inverted-F Antenna (PIFA) suitable for operation within the Medical Implant Communications Services (MICS) band. The novel antenna design incorporates a strategically positioned shorting pin, enhancing miniaturization and resulting in a more compact yet high-performance antenna. Simulations are conducted within a rectangular model, with the tissue properties mimicking those of the human body using bio-tissue materials. Subsequently, four different substrates are employed, maintaining constant antenna dimensions, to assess additional critical antenna characteristics such as return loss and gain. The antenna's performance is then compared across these different substrates. The designed antenna boasts a compact volume of 1.492 cm3, making it well-suited for successful integration into IMDs, thereby contributing to the advancement of medical technology and patient care.