I don't know, but I would guess that it means they can't tell enough from the ultrasound to rule out cancer or to consider it likely, so they might have to do the biopsy to see. I think that is fairly common. I think it is only in a small number of cases that the ultrasound can rule out cancer. I know I had to have an FNA biopsy and the doctor said there was nothing to suggest it was cancerous, but it also didn't rule it out.

In my case, it did turn out to be cancer, but I honestly think the doctors were slightly surprised. It is more common for it not to be.

Thyroid cancer is also...well, my consultant's words were "it's not the worst case in the world."