Sentence:  It has been suggested that the plasma thyroxine binding proteins , which have an extremely high affinity for thyroxine , compete with <WSD>thyroglobulin</WSD> for thyroxine ( Ingbar and Freinkel , 1957 ) . 
Meaning of the word marked with <WSD> tags:  an iodine containing protein that is obtained from the thyroid gland and exhibits the general properties of the globulins