This study presents a comprehensive theoretical assessment of lanthanide-based core-shell nanoparticles for theranostic applications in the near-infrared region. The complex refractive indices of fifteen lanthanide elements were calculated using density functional theory and the hybrid HSE06 method, which provided optical data in the spectral range of 100-2500 nm. Mie's theoretical calculations for various biocompatible coatings (SiO, PEG, TiO) indicated that TiO coatings exhibit better absorption efficiency in the NIR-I window (750-900 nm) (Q up to 9.4×10). The optimal core size for TiO-coated nanoparticles was determined to be between 90 and 110 nm, and the core-shell ratio ranged from 0.54 to 0.63, which provides the highest adsorption efficiency. Modeling biological heat transfer using the Pennes model with the finite element method revealed that the thermal reactions of tissues vary significantly for each tissue type. The thermal threshold time followed a power relation with . The fastest thermal response was observed in breast tissue (threshold time: 2.40-9.60 seconds), while the slowest response was noted in liver tissue (threshold time: 19.20-4.80 seconds). These findings offer key parameters for optimizing lanthanide-based therapeutic imaging platforms and provide a theoretical framework for predicting their performance in biological environments.