The procedure for the cytochemical bioassay of parathyroid hormone has been examined to improve the reliability and robustness of the assay. It has been shown that the control of the pH, at all stages of the assay, is essential. The use of the assay confirmed that the circulating level of the biologically active form of this hormone is less than one tenth of the values found by the more recent immunoassays, and is in good agreement with previously calculated values. The assay clearly distinguished between normal, hypoparathyroid and hyperparathyroid levels and allowed relative potencies to be ascribed to preparations of the hormone obtained from human, bovine and porcine sources.
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