Objectives: To evaluate the shielding effect of thyroid collar for digital panoramic radiography. Methods: 4 machines [Orthopantomograph (R) OP200 (Instrumentarium Dental, Tuusula, Finland), Orthophos CD (Sirona Dental Systems GmbH, Bensheim, Germany), Orthophos XG Plus (Sirona Dental Systems GmbH) and ProMax (R) (Planmeca Oy, Helsinki, Finland)] were used in this study. Average tissue-absorbed doses were measured using thermoluminescent dosemeter chips in an anthropomorphic phantom. Effective organ and total effective doses were derived according to the International Commission of Radiological Protection 2007 recommendations. The shielding effect of one collar in front and two collars both in front and at the back of the neck was measured. Results: The effective organ doses of the thyroid gland obtained from the 4 panoramic machines were 1.12 mu Sv for OP200, 2.71 mu Sv for Orthophos CD, 2.18 mu Sv for Orthophos XG plus and 2.20 mu Sv for ProMax, When no thyroid collar was used. When 1 collar was used in front of the neck, the effective organ doses of the thyroid gland were 1.01 mu Sv (9.8% reduction), 2.45 mu Sv (9.6% reduction), 1.76 mu Sv (19.3% reduction) and 1.70 mu Sv (22.7% reduction), respectively. Significant differences in dose reduction were found for Orthophos XG Plus and ProMax. When two collars were used, the effective organ doses of the thyroid gland were also significantly reduced for the two machines Orthophos XG Plus and ProMax. The same trend was observed in the total effective doses for the four machines. Conclusions: Wearing a thyroid collar was helpful when the direct digital panoramic imaging systems were in use, whereas for the indirect digital panoramic imaging systems, the thyroid collar did not have an extra protective effect on the thyroid gland and whole body.