An ultrasound is the next step to further characterize the nodule -- and about 50% of the time when there is a palpable nodule, there is more than one nodule found on ultrasound. The other test you need is a TSH. If this is normal and the nodule is >1cm, then the next step is fine-needle aspiration biopsy (NOT an I-123 scan).
90-95% of nodules are benign. Synthroid treatment is not indicated unless there is hypothyroidism (high TSH). These benign nodules can be watched with ultrasound.
Don't panic. Your doctor just wants to get a clear idea of what the nodule consists of and if it is compressing any neck structures.
Many of us with thyroid nodules do not have abnormal thyroid labs. Did your doctor tell you your thyroid levels were off? If so, can you post them here?