<p>Wireless body area networks (WBANs) consist of bio-sensor nodes that can be either wearable or implantable in the human body. These sensors play a crucial role in monitoring patients’ conditions both inside and outside hospitals. However, the sensor nodes in WBANs have limitations in energy, memory, temperature tolerance, and computational capability. Consequently, designing an efficient routing protocol for WBANs presents a significant challenge. It is essential to develop a routing protocol that effectively addresses these limitations. In recent years, many routing techniques have been proposed for WBANs; however, they have not adequately addressed critical aspects such as energy efficiency, stability, traffic load balancing, and temperature control in sensor nodes. To address these issues, this paper proposes an energy-efficient and thermal-aware routing protocol (SETA) based on an improved ant colony optimization algorithm for multi-WBAN environments. The SETA protocol employs a multi-effective cost function to select optimal forwarder nodes for constructing routes between the source sensor nodes and the sink node. Additionally, the proposed scheme introduces a new mechanism to dynamically change forwarder nodes after transmitting a certain number of data packets. This mechanism helps balance energy consumption and traffic load among nodes while preventing the forwarding of data packets from hotspot nodes. Furthermore, the SETA routing scheme considers both single-hop and multi-hop communication based on the state of the sensor nodes and the priority of the data that needs to be sent to the sink node. The findings indicate that the proposed SETA protocol achieves significantly better improvement rates in terms of packet delivery ratio, throughput, average end-to-end delay, energy consumption, convergence time, network lifetime, and temperature overhead compared to the existing PLQE, AMCRP, and THAT routing protocols.</p>