Thyrocalcitonin radioimmunoassay

Hypocalcemic perfusion of the pig thyroid in situ results in a slight rise in the systemic blood calcium level, which might be caused by a shutting-off of thyrocalcitonin secretion (C5). The normal level of thyrocalcitonin output might depend on the prevailing level of secretion of parathyroid hormone this possibility is supported by the observation that hypocalcemic perfusion of the thyroid in parathyroidectomized pigs was unaccompanied by the rise in systemic plasma calcium observed in intact animals. Until sensitive immunoassay procedures become more widely available, however, it is difficult to be sure whether or not thyrocalcitonin is secreted under resting conditions in the intact animal. A preliminary report of a radioimmunoassay technique for thyrocalcitonin, by Arnaud and Littledyke (A5), indicates that all plasma samples examined contained detectable thyrocalcitonin, and that thyroidectomy resulted in a decrease in thyrocalcitonin levels below those found in normal animals. 

The investigation of possible clinical disturbances of thyrocalcitonin secretion is likely to require measurement of the amount of the hormone present in body fluids. Since thyrocalcitonin is a peptide of as yet uncertain structure (see Section 7 below), and since the amounts present in blood are likely to be minute, its direct chemical determination may prove to be impracticable. Other peptide hormones, however, have been successfully measured in blood plasma by radioimmunoassay methods, and similar measurements of thyrocalcitonin have already been attempted (A5). 

Although techniques of this kind have been successfully applied to several peptide hormones in blood plasma, considerable problems must be overcome before a radioimmunoassay for thyrocalcitonin can be established as of adequate specificity and sensitivity for routine use. The relatively small molecular size of purified thyrocalcitonin peptide (see... 

Arnaud and Littledyke (A5) and Hargis et al. (HI) overcame the problem of antigenicity by conjugating relatively pure porcine thyro-calcitonin to rabbit albumin before injection, and claim that the antisera obtained were specific to thyrocalcitonin, judged by gel diffusion and immunoelectrophoresis results. The former workers have briefly described the use of this antiserum in a radioimmunoassay procedure similar to that devised by Berson et al. for parathormone (B4). They claim that the technique will detect 0.5 ng of thyrocalcitonin/ml of serum. These workers found no difference in the immunological behavior of human and porcine hormone, and quote a normal range of 30-85 ng/ml in human plasma. In animal studies, elevated values were found in older animals and in response to hypercalcemia. 

Although other workers have prepared and used antisera in immuno-fiuorescence studies, confirmatory reports of the practicability of preparing antisera suitable for radioimmunoassays have not yet appeared. A considerable degree of cross-reactivity between species, both in the effects of administered hormone and in hormone-antibody interactions, appears to be a feature of most thyrocalcitonin preparations so far studied. Although this partial lack of species specificity should facilitate the development of antisera and of isotopically labeled hormone preparations from animals for use in immunoassays in man, it must also leave some doubt with respect to the specificity of such assays when applied to so complex a mixture as blood plasma. 

Quite apart from the feasibility or otherwise of synthesizing peptides possessing thyrocalcitonin-like activity, the amino acid composition of the molecule is of interest in that the preparation of iodinated hormone of high specific activity is necessary if radioimmunoassay is to be attempted. 

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See also in sourсe #XX -- [ ]

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