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The changes of the serum protein-bound iodine during growth in the Holstein calf and the Wistar albino rat Cheeke, Peter Robert

Abstract

The changes in the level of circulating thyroid hormone, or protein-bound iodine (PBI), in the male Holstein calf and the Wistar Albino rat during a portion of the growth period have been determined. Two levels of nutrition were employed.in each case to determine if the nutritional status exerted an appreciable effect on the measured PBI level. Repetitive measurements of the serum PBI were conducted with each of eight calves in order to assess age changes, while with the rats single determinations were made on animals killed at regular weight intervals. Carcass analysis of the rats, including body fat and water determinations of the individual animals, and protein and ash analysis of a pooled sample from each group, was conducted with the intent of relating age changes of the serum PBI to a measure of "metabolically active tissue". The PBI level of the Holstein calf was found to increase with age in the growth interval considered. The rate of change of the PBI level of the low plane calves appeared to be less than in the high plane animals; the degree of variability was such, however, that a definite effect of nutrition could not be shown. No correlation was found between either the initial PBI level or the level at slaughter and the daily rate of gain. The resting metabolic rate of the calves was determined prior to the removal of a blood sample for PBI analysis. No relationship between the serum PBI level and the metabolic rate existed. In the case of the rats, no correlation was found between the serum PBI level and chronological age, body weight or fat-free body weight. A great deal of variability of the PBI level among animals in the same group was observed. Possible reasons for this variability are discussed. The body composition data of the high plane rats followed accepted trends, with the body water, protein and ash fractions exhibiting differential growth with respect to body weight and fat-free body weight. The composition of the low plane rats was not appreciably different from that of the high plane animals at equal body weights. The realimentation period was characterized by the deposition of large quantities of body fat. The significance of these findings in terms of the results of other investigators is discussed. Various workers have attempted to relate the serum PBI level of a young animal with its potential productive worth. No evidence was obtained in this study of such a relationship; a discussion of the many factors affecting the serum PBI level is offered to support the contention that no such relationship should be anticipated.

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