Sentence:  Whether this abnormal TSH differs chemically from pituitary TSH , or is , alternatively , normal TSH with its period of effectiveness modified by some other blood <WSD>constituent</WSD> , cannot be decided without chemical study of the activity in the blood of these patients and a comparison of the substance responsible for the blood activity with pituitary TSH . 
Meaning of the word marked with <WSD> tags:  an artifact that is one of the individual parts of which a composite entity is made up; especially a part that can be separated from or attached to a system