Neither the report by Meyer and Abdel-Bari regarding thyrocalcitonin-like activity in an extract derived from a medullary carcinoma of the thyroid gland (March 7, 1968) nor the editorial summarizing the possible roles of thyrocalcitonin in relation to disease mentioned the most recent report of Williams and his associates. Their experience suggests that appreciable amounts of (PGs) prostaglandins were present in tumor tissue in 4 of 7 cases of medullary carcinoma of the thyroid. Elevated blood levels of PGs were detected in 2 of these patients--both of whom had diarrhea. Although the patient of Meyer and Abdel-Bari was not reported to have had this particular symptom, diarrhea has been described in association with a significant number of medullary carcinomas. The group of long-chain fatty acids classified as PGs has myriads of diverse activites in biologic systems. Stimulation of intestinal smooth muscle is 1 of these, and may be the reason for the otherwise unexplained intestinal hyperactivity. Also, since the assay for thyrocalcitonin is nonspecific, it is conceivable that some of the PGs have a similar acute hypocalcemic and hypophosphatemic effect, although this has apparently not been reported to date.