The overall interobserver reproducibility of thyroid fine-needle aspiration (FNA) has not been comprehensively assessed. A blinded 6-rater interobserver reproducibility study was conducted of 200 thyroid FNA cases using the UK System, which is similar to The Bethesda System for Reporting Thyroid Cytology: Thy1, nondiagnostic; Thy2, nonneoplastic; Thy3a, atypia, probably benign; Thy3f, follicular lesion; Thy4, suspicious of malignancy; and Thy5, malignant. There was good interobserver agreement for the Thy1 (κ = 0.69) and Thy5 (κ = 0.61), moderate agreement for Thy2 (κ = 0.55) and Thy3f (κ = 0.51), and poor agreement for Thy3a (κ = 0.11) and Thy4 (κ = 0.17) categories. Combining categories implying surgical management (Thy3f, Thy4, and Thy5) achieved good agreement (κ = 0.72), as did combining categories implying medical management (Thy1, Thy2, and Thy3a; κ = 0.72). The UK thyroid FNA terminology is a reproducible and clinically relevant system for thyroid FNA reporting. This study demonstrates that international efforts to harmonize and refine thyroid cytology classification systems can improve consistency in the clinical management of thyroid nodules.