If a solid heterogeneous nodule on the thyroid is benign and not causing tsh levels to change but is growing rapidly, what is it?

Thyroiditis?: Make sure you see an endocrinologist for further evaluation. Thyroid antibodies has to be measured.

Something to monitor: A fna only gets to the parts of a nodule the needle enters and may miss a problem in an area of the nodule it doesn't enter. Therefore, if a nodule is rapidly growing it should be closely monitored by an endocrinologist who will help determine when there should be a concern.

Re-evaluation: If a nodule is "growing rapidly", it will need to be re-evaluated. Most likely it will need to be re-biopsy. Most of the time, a cancer can just be a speck in the nodule instead of the whole nodule so a biopsy might have missed it the first time.