An Improved Method for the Biological Assay of Thyrocalcitonin

Abstract

The method for biological assay of thyrocalcitonin introduced by Hirsch et al. (Science 146 412, 1964) was investigated for possible improvements in the interest of greater convenience and higher precision. The original method was based on the hypocalcemic effect of thyrocalcitonin 1 hr. after subcutaneous injection of thyrocalcitonin into intact Holtzman rats (6 1/2 weeks old) that had been fed a special low Ca diet for 4 days. It was found that feeding the special diet for 1 instead of 4 days increased the slope of the dose-response line. The slope was also steeper in rats fed the special diet 1 day than in rats fasted 1 day. The rats responded to lower doses of thyrocalcitonin when they were fed the special low calcium diet that when they were fed a commercial diet that was low in P as well as Ca. Holtzman rats responded to lower doses of thyrocalcitonin than Charles River on Blue Spruce rats. In 5-week-old Holtzman rats the response to thyrocalcitonin was greater and the slope of the dose-response line was steeper than in older rats. On the basis of these findings 2 modifications were made in the original procedure: intact Holtzman rats were used at 5 instead of at 6 1/2 weeks of age, and the rats were fed the special low Ca diet (adequate in P content) for 1 day instead of 4 days. The mean index of precision ([lambda]), using the modified procedure, was 0.15, significantly better than the 0.23 reported intially and the 0.20 obtained more recently with the original method.