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 than 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 or 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: 1) intact Holtzman rats were used at 5 instead of at 6 1/2 weeks of age, and 2) 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 (λ), using the modified procedure, was 0.15, significantly better than the 0.23 reported initially and the 0.20 obtained more recently with the original method. (Endocrinology81: 610,1967)