Doctor said thyroid is almost dead…

My THYROGLOBULIN is 2.9 ng/mL which my doctor said means it’s nearly dead. She said if it hits 2.8 or lower, she would recommend removing my thyroid. I was a bit taken aback by this. Does a non-functioning thyroid really need to be removed? Ultrasounds have always come back fine. I was looking back at my labs over the past 10 years, and when I was first diagnosed, my THYROGLOBULIN was 9.1. I’m curious: what causes THYROGLOBULIN to decrease?