Objective: Outcomes of patients with cytologically indeterminate thyroid nodules not referred for thyroidectomy have hardly been investigated. We previously reported outcomes of 322 patients with thyroid nodules classified according to the Bethesda System of Reporting Thyroid Cytology (BSRTC) as indeterminate (B3/B4), of whom 123 (38.2%) underwent thyroidectomy. In the present extension study, we investigated adherence and outcomes in the remaining unoperated 199 patients. Methods: We conducted a file review of 189/199 patients with thyroid nodules cytologically diagnosed as B3 ( n = 174) or B4 ( n = 15) in 2011–2012 who were conservatively followed at our institution until 2019. Results: Among 174 patients with B3 nodules, 140 (80.4%) underwent repeated ultrasound. Nodular growth was detected in 23 (16.4%), and findings remained stable in 105 (75%). Fine-needle aspiration was repeated in 88/174 patients (50.6%), with B2 results in 62 (70.4%) and B3/B4/B5 in 20 (22.7%). Thyroidectomy was performed in 14/174 patients (8%) in the B3 and 5/15 patients (33%) in the B4 group at a median of 5 years’ follow-up; thyroid cancer was diagnosed in 4/14 patients (28.5%) and 3/5 patients (60%), respectively. For B3 patients who remained unoperated, none had evidence of thyroid cancer at last follow-up. A reason for avoiding surgery was documented in 6/10 unoperated B4 patients (1 thyroid lymphoma, 3 died of unrelated causes, 2 were considered inoperable due to advanced age). Conclusions: Most patients with initially unoperated B3/B4 nodules adhere, at least partially, to active surveillance. For B3 nodules, subsequent thyroidectomy and thyroid cancer detection are rare events, and patients may be safely managed without using molecular markers. Thyroid cancer is diagnosed in most B4 patients who undergo thyroidectomy in our institution.