Sentence:  It has been suggested that the <WSD>plasma</WSD> thyroxine binding proteins , which have an extremely high affinity for thyroxine , compete with thyroglobulin for thyroxine ( Ingbar and Freinkel , 1957 ) . 
Meaning of the word marked with <WSD> tags:  the colorless watery fluid of the blood and lymph that contains no cells, but in which the blood cells (erythrocytes, leukocytes, and thrombocytes) are suspended