More than likely just an excuse to say they are backlogged and haven't done the report yet. The 'measurements' are done on the day by the radiographer, and they are there to see on the actual ultrasound itself.
If you remember the name of the place you had the US done, I would be giving them a call and just ask, have you sent my doctor the report of the US I had done on blah blah date? They will tell you yes or no. You would be surprised to learn that many doctor's have very lax practises and rarely follow up on results and presume they are just not back yet. Believe me as a long term patient I have had it happen to me too many times!
Be assertive and ask the US and then call the doctor again.
I would venture a guess you have nodules on your thyroid (don't worry many people do), and they are measuring the size of them. If they are large, some form of biopsy would be in order. If they are small, no biggie :)