A 70 year old man with toxic multinodular goiter. He was treated with radioiodine therapy. Two years later, hypothyroidism developed. He was on thyroxine. In the thyroid clinic thyroid function shows the following results:

FT3 - 1.3 pmol/L (normal 1.1 to 3.2).

FT4 - 160 pmol/L (normal 9 to 23).

TSH – 1.5 mIU/L (normal 0.3 to 5).

GP asked the lab for the label used in competitive binding assay to measure FT4

What is the best answer for the GP