Context: Thyroid nodule size is routinely measured, although its impact on thyroid cancer risk is unclear.
Objective: Our objective was to evaluate the association of nodule size upon cancer risk.
Design, setting, and patients: We conducted a retrospective cohort analysis at an academic hospital with 4955 consecutive patients evaluated between 1995 and 2009.
Intervention: Ultrasound and ultrasound-guided fine-needle aspiration of nodules >1 cm was done. Indeterminate and malignant nodules were referred for surgery, and histopathology was reviewed.
Main outcome measure: The presence and histological subtype of cancer was evaluated.
Results: Of 7348 evaluated nodules, 927 (13%) were cancerous. Of those 1.0 to 1.9 cm in diameter, 10.5% were cancerous. In contrast, of those >2.0 cm, 15% were cancerous (P < .01). However, nodules 2.0 to 2.9, 3.0 to 3.9, and >4 cm were cancerous in 14%, 16%, and 15% of cases (P = .14), respectively, demonstrating no graded increase in risk beyond the 2-cm threshold. When malignant, the proportion of papillary carcinoma decreased (nodules 1.0-1.9 cm, 92% of cases; 2.0-2.9 cm, 88%; 3.0-3.9 cm, 83%; >4 cm, 74% [P < .01]), while follicular carcinoma increased (1.0-1.9 cm, 6%; 2.0-2.9 cm, 7%; 3.0-3.9 cm, 12%; >4 cm, 16% [P < .01]) as nodules enlarged. Nodules size did not influence cytology distribution or risk of false-negative aspirates.
Conclusions: Increasing thyroid nodule size impacts cancer risk in a nonlinear fashion. A threshold is detected at 2.0 cm, beyond which cancer risk is unchanged. However, the risk of follicular carcinomas and other rare thyroid malignancies increases as nodules enlarge.