Thyroid and allergy tests normal. Ct scan shows nasal polyps. Ent suggests operating but no other treatments explored. Need 2nd opinion or just do it?

Second opinion. There are several causes of nasal polyps including allergic fungal sinusitis, cystic fibrosis, and in patients with Aspirin "allergy" and asthma. It may help to see an allergist to attempt to identify these underlying conditions. Also, Prednisone is a strong steroid medication that may shrink nasal polyps without surgery. My opinion-always get second opinion before surgery.
Why not get opinion? The only time i think it's wrong to get a second opinion is when it delays much-needed treatment. If this isn't a life-and-death emergency, make an appointment with another ENT and get their opinion. This way, you may learn about other approaches to your condition (if they exist) - and you may also feel more confident about your own doctor's recommendation. It's a surgery, after all! good luck.
Get a second. Surgery for any reason is a serious choice. For a non emergent reason, a second opinion is always a good idea.