7mm thyroid nodule seen on ultrasound, then 1 month later, it was gone. how can this be?

Nodules: Thyroid modules are common. Nodules < 10 mm in size are almost universally benign. I suspect the appearance and disappearance of the nodule is a technical variation in technique, although nodules are capable of decreasing in size.

Options: The "nodule" could have been a cyst which involuted over time. Another option is that the first or second ultrasound was incorrectly read. A third option is the nodule may have gotten small enough to avoid being detected.