In the rapidly evolving field of the Medical Internet of Things (MIoT), ensuring the confidentiality and integrity of sensitive health data is critical. This paper presents a Modified Advanced Encryption Standard (AES) algorithm specifically designed for secure, real-time healthcare monitoring applications. The proposed enhancements include four major contributions: (1) dynamic S-box generation using Fisher-Yates shuffle seeded with device-specific values to increase unpredictability; (2) a hybrid Counter-Cipher Block Chaining (CTR-CBC) mode that combines high throughput with strong diffusion; and (3) lightweight key expansion using SHA-256, significantly reducing memory and computational overhead and (4) Cipher-based Message Authentication Code (CMAC) to provide integrity. Comparative analysis with standard AES demonstrates improved encryption speed, better adaptability to embedded environments, and increased resilience to cryptanalytic attacks, making it highly suitable for MIoT systems.