The thyroid enlargement could be caused by Hashimoto's, Graves Disease, hypothyroidism or hyperthyroidism.
There are no significant lesions or dominant masses and the report says there are no indicators of malignancy, therefore the chances of cancer would be, basically, zero... In other words, there is nothing significant about your report and nothing to worry about.
That said, what prompted you to have the ultrasound? Are you having symptoms of a thyroid problem? If so, what symptoms and what problem do you think you might have?
What, exactly, is your normal TSH level? Is that the only thyroid-related blood test you've had? There are other tests you should have that are more important than TSH... those include Free T4 and Free T3 as well as antibody tests appropriate to whatever symptoms, if any, you might have.
With further information, we can help you better.
There are a lot of things that affect TSH and cause it to fluctuate and the fact that you had skull based surgery could certainly have been an influence on it.
Your Free T4 indicates that your thyroid may be beginning to malfunction as it's somewhat low...
Have you had a thyroid ultrasound to check for nodules/swelling on your thyroid?
Also, cortisol testing would be advisable as high cortisol levels can cause the symptoms you report.