Table 1.

Eligibility criteria.

Participant Inclusion and Exclusion Criteria
Inclusion criteria	Exclusion criteria
•Age 18 years or older	•Thyroid cancer that is known to have extended beyond the thyroid (e.g. extrathyroidal extension, nodal or distant metastases)
•Newly diagnosed, untreated papillary thyroid cancer (PTC) < 2cm in maximal diameter on ultrasound imaging. The fine needle aspiration biopsy cytology of the primary tumor is required to be read as PTC or suspicious for PTC*	•A history of prior thyroid surgery*
•Absence of metastatic cervical lymphadenopathy on imaging (ultrasound the neck or other)	•The primary PTC is encroaching the recurrent laryngeal nerve or trachea (which is considered at potential high risk for invading these structures in the event of disease progression)
•No other absolute* indication for thyroid or parathyroid surgery at the time of the assessment	•Proven or suspected poorly differentiated or non-papillary thyroid cancer
•Patient must grant permission for review of thyroid cancer-related medical records	Medically unfit for surgery due to severe co-morbidity. Severe comorbidity may include another active malignancy with limited life expectancy of < 1 year*
	•Pregnancy at the time of study enrollment
	•One or more absolute indications for thyroid or parathyroid surgery (other than thyroid cancer)*
	•Patient is unable or unwilling to consent for study follow-up procedures (e.g. due to current severe active cognitive or psychiatric impairment, substance abuse, or other reasons)

*Changes in the original Toronto protocol, approved by the University Health Network Research Ethics Board (April 7, 2020) and incorporated in the pan-Canadian at onset of the study include: not mandating central cytopathology review (i.e. may be performed at the discretion of the investigator if there is any uncertainty or concern about the original review, but not mandating this due to limited value with some patient delays observed in the Toronto study), not excluding patients who have mild thyroid or parathyroid disease that does not meet established absolute indications for surgical treatment (e.g. hyperthyroidism under control with medication, mild primary hyperparathyroidism), excluding patients with any prior thyroid surgery (not just thyroid cancer surgery, as prior partial thyroidectomy could confound results at long-term follow-up, although no patients with prior thyroid surgery have been enrolled in the Toronto study), and permitting patients who have had a recent diagnosis, treatment, or active surveillance of another malignancy to participate in the study if the expected survival of the other malignancy is > 1 year (as patients with other malignancies, including those under active surveillance for other low risk cancers, have expressed interest in this study).