Thickness of lead in thyroid collar:
A thyroid shield can reduce the effective dose by 2.5 times and the total exposure by almost 50%. Therefore, thyroid shields should have at least 0.5-mm lead equivalent thickness for neck and thyroid protection.
Ref: Cheon BK, Kim CL, Kim KR, Kang MH, Lim JA, Woo NS, Rhee KY, Kim HK, Kim JH. Radiation safety: a focus on lead aprons and thyroid shields in interventional pain management. The Korean journal of pain. 2018 Oct;31(4):244.
Note: 0.2-0.5 mm is the recommended thickness for the thyroid collar.
Key Concept
The thyroid gland is more susceptible to radiation exposure during dental radiographic exams given its anatomic position, particularly in children. Protective thyroid collars and collimation substantially reduce radiation exposure to the thyroid during dental radiographic procedures. Because every precaution should be taken to minimize radiation exposure, protective thyroid collars should be used whenever possible (ADA 2012).
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition
0.5 mm
1.0 mm
1.5 mm
2.0 mm