In this study, the effect of Indian reference BOttle MAnnikin aBsorber (BOMAB) neck with axial cavity and American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom using pencil sources of ¹³³Ba(¹³¹I simulant) on counting efficiency (CE) is seen experimentally in static geometry for whole-body monitoring system comprising 10.16-cm diameter and 7.62-cm-thick NaI(Tl) detector. The CE estimated using the neck part of BOMAB phantom is 50.2 % lower in comparison with ANSI phantom. In rest of the studies FLUKA code is used for Monte Carlo simulations using ANSI/IAEA thyroid phantom. The simulation results are validated in static geometries with experimental CE and the differences are within 1.3 %. It is observed that CE for pencil source distribution is 3.97 % higher for ¹³³Ba in comparison with CE of ¹³¹I source. Simulated CE for pencil source distribution is 4.7 % lower in comparison with uniform source distribution in the volume of thyroid for ¹³¹I. Since the radiation workers are of different physique; overlying tissue thickness (OTT) and neck-to-detector distance play an important role in the calculation of activity in thyroid. The CE decreases with increase in OTT and is found to be 5.5 % lower if OTT is changed from 1.1 to 2 cm. Finally, the simulations are carried out to estimate the variation in CE due to variation in the neck-to-detector distance. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. In conclusion, the calibration of whole-body monitoring system for ¹³¹I should be carried out with ANSI/IAEA thyroid phantom, the neck-to-detector distance controlled or the CE corrected for this, and the CE should be corrected for OTT.