Doctoral Dissertations

Date of Award

3-1971

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

E. W. Swanson

Committee Members

J. T. Miles, M. J. Montgomery, S. L. Hansard, R. H. Feinberg

Abstract

Two groups of experiments were conducted to investigate the relationship of the thyroid gland and calcium metabolism in the bovine. The first group included investigations of the effects of thyroid status (calcitonin and thyroxine status) on the response of mature cattle and calves to hypercalcemia induced by intravenous infusion of calcium solutions and changes in serum calcium at parturition. Three unsupplemented athyroid cattle cleared infused calcium from the blood more slowly than three controls. However, five thyroiddamaged cows fed iodinated casein cleared excess calcium from blood as well as five controls. In the third experiment, six thyroid-damaged calves which were receiving slightly excessive thyroxine therapy cleared infused calcium from blood as well as controls. The results of these experiments indicated that thyroxine therapy restored the normal ability to counteract hypercalcemia. No defect logically attributable to calcitonin was evident. The hypercalcemic response of three thyroid-damaged calves and three controls was tested before and during thyroxine therapy to the thyroid-damaged calves. Unsupplemented thyroid-damaged calves cleared excess calcium from the blood as well as controls. When excessive thyroxine therapy was given to the thyroid-damaged calves their ability to counteract hypercalcemia appeared to be decreased. This was attributed to increased bone calcium turnover stimulated by excess thyroxine in the supplemented calves. Two hyperthyroid control calves cleared calcium from blood at a slower rate than two thyroid-damaged calves. Effects of excess thyroxine on bone calcium turnover could have been responsible for this occurrence. No consistent effects of thyroid status on serum phosphorus and magnesium were noted in these tests. The results did not demonstrate a major role for calcitonin in the hypercalcemic response of calves. Important effects of thyroxine could be inferred from the data. Blood samples were taken before calving, at calving, and after calving in six pairs of control and thyroid-damaged cows. A significant drop in serum calcium at calving was noted in controls but not in thyroid-damaged cows. This finding was attributed to release of calcitonin at parturition in the control cows. Hypocalcemic activity was present in the serum of a normal parturient cow, as judged by the decreased serum calcium in normal calves infused with such serum, but no such activity was present in the serum from a thyroid-damaged parturient cow. Calcitonin appeared to be involved in the serum calcium changes at parturition. In the second group, comprising six experiments, the effects of thyroid status on normal calcium metabolism in cows and calves were studied. In three experiments concurrent 5-day chemical and radiocalcium balances were conducted on calves dosed simultaneously with 47Ca intravenously and 45Ca orally. Bone uptake and gut retention were determined. Lower calcium absorption in three thyroid-damaged calves compared to three controls was indicated in one experiment. Low calcium absorption due, perhaps, to hyperthyroidism in two controls in another experiment could have masked decreased calcium absorption in two thyroid-damaged calves. When excessive thyroxine therapy was given to three thyroid-damaged calves no difference from absorption by three controls was noted. The results indicated that increasing the level of circulating thyroxine increased the turnover rate of body calcium. In two experiments a total of five lactating cows were dosed intravenously with radiocalcium to study the effects of thyroxine status on calcium metabolism in lactation. In both experiments increased circulating thyroxine levels, whether induced by injection of thyrotrophin or by direct thyroxine injections, increased the mobilization of body calcium reserves. The transfer of calcium from dam to fetus in three control and three thyroid-damaged cows was not affected by thyroid status.

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