<p>Secure sharing of patient data remains a key challenge in IoT-based healthcare systems, where wearable devices generate continuous medical data. Traditional encryption-based approaches and conventional blockchain platforms struggle with scalability, transaction latency, and computational overhead, limiting their suitability for large-scale healthcare environments. To address these limitations, this paper proposes a Secure Healthcare Data Transmission Mechanism (SHDTM) built on the IOTA distributed ledger. The scheme utilizes IOTA’s Directed Acyclic Graph (DAG) architecture for parallel, feeless transaction validation and integrates encrypted data streaming through Masked Authenticated Messaging. To further reduce retrieval delays, a MongoDB-based indexing deployed on a fog server maps patient identifiers and metadata to ledger addresses, enabling rapid data lookup without scanning the entire network. InterPlanetary File System (IPFS) storage is incorporated to efficiently manage large medical files while preserving integrity through cryptographic hashing. Experimental evaluation using Android-based wearable devices and a Raspberry Pi demonstrates significant performance improvements over existing approaches. SHDTM reduces connection invitation time by up to 70.91% and concurrent transaction latency by 69.38% compared to BDIMHS. For data registration, transaction time is reduced by up to 72.66% in sequential scenarios and 86% under concurrent load. The proposed framework also lowers CPU utilization and memory consumption, making it suitable for resource-constrained IoT healthcare environments. These results indicate that SHDTM provides a scalable, low-latency, and cost-efficient solution for secure healthcare data sharing, supporting real-time medical monitoring while preserving patient privacy and system efficiency.</p>