What determines when thyroid nodules get biopsied v. waiting a period of time(perhaps 3-6 mos.) and repeating ultrasound?

Several things : When to biopsy thyroid nodules is based on the clinical judgement of your doctor. Risk factors include history of ionizing radiation to the neck, family history of thyroid ca. Factors such as nodule size, whether dominant nodule is present, especially if solitary rather than as part of multinodular goiter and if calcification is present are important. ? Local symptoms = evaluation.

Further testing: Thyroid function should be evaluated by blood tests, and ultrasound & nuclear scan then determination can be made regarding the potential for malignancy and, therefore, biopsy.