1. Administration of a large amount of unlabelled calcium orally 60 min after intravenous injection of 47Ca leads to a temporary rise of serum specific radioactivity in healthy volunteers.

2. This short-term rise is followed by a more rapid decline and in some instances by oscillations of specific radioactivity.

3. Both these effects could be caused by secretion of thyrocalcitonin resulting from a rise of serum calcium concentration during calcium absorption from the gastrointestinal tract.

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