I had FN Biopsy before I had my Thyroid removed and I ate and drank 3 hours before FN biopsy and I had no numbing agent not by choice....all places are different and maybe they are afraid of him vomiting during the FN biopsy.
Sorry again, is he having Thyroidectomy or FN biopsy? I would not do surgery until after the FN Biospy ....that would be a crazy thing to do not having a thyroid is a nightmare hormone wise.
I'm pretty sure I had something drink minutes before I went in for my FNB, and had no problems. I had two thyroid surgeries where I was under general anesthesia, and I wasn't allowed to eat or drink after midnight for those.
As for the odds that a cystic nodule is cancerous - I don't know. I know 90% of thyroid nodules are benign. The size (>4cm) and irregular borders put it into a more suspicious category. Also, if it is a mix of cystic and solid components (which an irregular border with nodularity might suggest), that makes it more suspicious too.
Also - I've never had a cystic nodule so I don't know what they do during the biopsy, but maybe the no eating/drinking guidelines come in to play if they plan on draining the cyst? My nodules were all solid, so I don't know what they do for cysts - if it is fluid filled I know they sometimes have problems getting enough cells to analyze and when that happens you can get an inconclusive result.
Good luck with your son's biopsy.
From the Mayo Clinic...
Common types of needle biopsy techniques include:
Fine-needle aspiration. This type of needle biopsy uses a thin, hollow needle to draw cells from your body.
Core needle biopsy. This type of needle biopsy uses a wider needle than does fine-needle aspiration. The needle used during a core needle biopsy is a hollow tube that allows the doctor to extract a core of tissue for testing."
As the core needle is larger...ouch!...it's usually done under local anesthesia. You cannot eat or drink prior to this procedure.