What Happens after an Incidental Thyroid Nodule? Patterns of Thyroid Ultrasound Use and Downstream Evaluation

BACKGROUND: Incidental thyroid nodules (ITNs) detected on nonthyroid imaging contribute to thyroid cancer overdiagnosis, largely through downstream evaluation initiated by thyroid ultrasound (TUS). However, TUS follow-up rates for ITNs and their drivers in routine practice remain unclear. We quantified TUS utilization after ITN detection and identified patient, imaging, nodule, and reporting factors associated with this first step in the diagnostic cascade. METHODS: We conducted a retrospective cohort study of adults without prior thyroid disease who underwent imaging that incidentally captured the thyroid at Mayo Clinic sites (2017-2023). A validated natural language processing pipeline identified ITNs and extracted nodule characteristics from radiology reports across multiple imaging modalities. The main outcome was the rate of TUS within 180 days after ITN detection. Clinical outcomes, including thyroid biopsy, thyroidectomy, and thyroid cancer diagnosis, were compared between patients who underwent TUS and those who did not. Factors associated with TUS utilization were assessed using multivariable logistic regression with penalized variable selection. RESULTS: Among 115,683 patients who underwent thyroid-capturing imaging, 8426 (7.3%) had an ITN. These patients were predominantly female (64.6%) and White (89.7%), with a mean age of 61.7 years (standard deviation 15.2). Most nodules were detected on computed tomography (80.8%), most commonly from chest imaging (52.3%). Overall, 1677 patients with ITNs (19.9%) underwent TUS. In multivariable analysis, TUS use was independently associated with higher body mass index (odds ratio [OR], 1.20 per 10-unit increase; confidence interval [CI] [1.08-1.33]), nodule size ≥1.5 cm (OR, 1.79; CI [1.52-2.10]), and a radiologist recommendation (OR, 2.51; CI [2.12-2.98]), whereas older age was associated with lower odds of TUS (OR, 0.89 per 10-year increase; CI [0.84-0.93]). Patients undergoing TUS had higher rates of thyroid biopsy, partial thyroidectomy, and total thyroidectomy compared with those without it. All detected thyroid cancers (103 cases) occurred in the TUS group, representing 6.1% of that cohort. CONCLUSIONS: One in five patients with ITNs underwent TUS, representing a critical gateway to downstream diagnostic procedures and thyroid cancer detection. Factors associated with TUS utilization highlight opportunities to better define appropriateness of ultrasound after incidental nodule detection and to reduce unnecessary testing and treatment.