The present research aimed to assess the accuracy and precision of the TheraMon microsensor embedded in different thicknesses of Hawley retainers (HR) for comparison with vacuum formed retainers (VFR). Thirty microsensors contained within different thicknesses and composition of retainers were divided into three equal groups: Group A thick coverage HR (3 mm), Group B thin coverage HR (1 mm), and Group C VFR (1 mm). The microsensors were immersed in thermostatic water at a controlled temperature of 35°C, which corresponds to the average intra-oral temperature. After 1 week, data were gathered using the TheraMon client software and analysed using ANOVA and Turkey’s HSD tests. All TheraMon® microsensors were functional and produced uninterrupted recordings during the 1-week test period. Thermal detection differed between the three removable retainer groups. A near accurate thermostatic water detection was noticed with the thin HR with a mean temperature of 34.81 ± 0.04°C, followed by VFR 34.77 ± 0.09°C, and finally the thick HR 34.73 ± 0.05°C (ANOVA -value = 0.025). A between-group comparison showed a significant mean difference (MD) between the thin and thick HR groups (MD: 0.08, -value = 0.01). However, there were no significant differences between VFR and neither the thick Hawley (MD: 0.04, -value = 0.27) nor the thin Hawley group (MD: -0.03, -value = 0.39). A removable retainer’s variation in material thickness and composition could induce small but detectable changes in the precision of thermal detection by TheraMon microsensors.