Background: In 131I therapies internal dosimetry is crucial for determining the mean absorbed dose to organs at risk, particularly the bone marrow, which has a dose constraint of 2 Gy. Traditionally, multicompartmental models have been used for bone marrow dosimetry, necessitating whole-body absorbed-dose assessments. However, noninvasive techniques, such as γ-camera scans or ceiling-mounted Geiger–Müller (GM) counters, can estimate the aforementioned. This study was aimed to evaluate the agreement between whole-body mean absorbed dose using γ-camera scans and ceiling-mounted GM in patients with thyroid carcinoma undergoing 131I therapy. Methods: This study included 31 patients with thyroid cancer who were treated with 131I. The whole-body time-integrated activity (TIA) and mean absorbed dose were estimated using the elimination curves obtained with γ-camera scans and ceiling-mounted GM. In addition, statistical analysis was performed on the data to determine the Coefficient Correlation Coefficient and the Bland–Altman limits of agreement for both parameters, as well as for the elimination curves' effective half-life. Results: The study revealed correlations of 0.562 and 0.586 between whole-body TIA and mean absorbed dose, respectively. The Bland–Altman limits of agreement were found to be below −3.75% and within 12.75% of the bone marrow dose constraint of 2 Gy. The nonparametric evaluation revealed that whole-body TIA and mean absorbed dose medians from GM were lower than those from γ-camera scans (p < 0.001). Effective half-life estimation mean was significantly lower in the GM than in the γ-camera of 13 and 23 h. Conclusions: Although GM calculates the whole-body absorbed dose with margins of error within clinical acceptance, underestimation of the effective half-life makes it an unacceptable substitute method for γ-cameras in clinical practice. Further research should be conducted to evaluate single-point GM measurement substitutions in time–activity curves. [ABSTRACT FROM AUTHOR]