<p>Wireless sensor networks (WSNs) have gained significant attention in recent research due to their potential in various applications. However, energy conservation remains a critical challenge, as the nodes within these networks operate on non-replaceable batteries. This paper introduces a cluster-based proactive routing protocol designed for three-tier energy-heterogeneous WSNs, aiming to enhance network lifetime and energy efficiency. The proposed protocol incorporates a modified threshold-based cluster head (CH) selection mechanism, which considers both the energy levels and distances of sensor nodes (SNs) relative to the base station (BS). This approach prioritizes nodes with higher energy and closer proximity to the BS, increasing their likelihood of being elected as CHs. Furthermore, this protocol minimizes the formation of unnecessary CHs, particularly for SNs with low remaining energy, thereby reducing energy waste. Moreover, the proposed protocol is modified to address the challenge of long transmission distances for low-power nodes by strategically deploying them in specific network zones, thus further optimizing energy efficiency. Simulation results demonstrate that the modified protocol outperforms existing protocols such as SEP, IMPLEACH, and EEECA-THWSN in terms of total network lifetime, stability period (first node death), half-node death, last node death, throughput, and normalized remaining energy. Numerically, the proposed protocol with normal zone improves network performance, achieving an increase of up to 27% in network lifetime and throughput compared to other protocols.</p>