To describe the protocol and progress of a thyroid study using thyroid ultrasonography in emergency workers who responded to the Fukushima nuclear accident.

Thyroid ultrasonography was performed on Fukushima emergency workers at over 60 health examination institutions. The accuracy of ultrasonography is controlled by standard procedural protocols, examiner training, and a central review system. Thyroid findings are classified into 4 categories: Category A1 (no nodule or cyst), Category A2 (nodules ≤ 5.0 mm and/or cysts ≤ 20.0 mm), Category B (nodules ≥ 5.1 mm and/or cysts ≥ 20.1 mm), and Category C (requires immediate further examination). Participants classified as Categories B or C are recommended for secondary examination.

Among 3398 participants with available ultrasound images obtained at the first health examination between January 2016 and October 2023, 45.2 % were classified as Category A1, 39.2 % as Category A2, 15.5 % as Category B, and 0 % as Category C. Of the 207 participants for whom secondary examination results were available, seven were diagnosed with cancer or suspected cancer.

An accuracy control system of thyroid ultrasonography has been established which will continue to carefully investigate the thyroids of Fukushima emergency workers.