The Effect of Hypothyroidism on the Metabolism of Carotene in Dogs

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Abstract
In normal and thiouracil-treated young mature dogs comparable increases in liver vitamin A concentrations were observed following the ingestion of carotene. Thiouracil administration caused, however, a rise in blood vitamin A concentration as compared with those found in normal controls. The increase was accompanied by similar changes in serum cholesterol levels and by the appearance of significantly increased serum carotene. Absorption of a single dose of carotene was reflected in normal dogs by a peak in the vitamin A level between the 8th and 11th hour following the dose. The magnitude of the change over the predosing level was not significantly altered in hypothyroidism. Only traces of vitamin A could be demonstrated in the urine of either normal or hypothyroid dogs fed carotene with the exception of an instance where the blood level had exceeded 1,000 μg% of vitamin A following thiouracil treatment. It is suggested that the circulating level of vitamin A was too low after carotene feeding to cause excretion in the urine. Following carotene administration both hypothyroid and normal dogs had considerable quantities of carotene in their livers. Carotene and vitamin A were present in the adrenals of carotene-fed dogs in nearly equal amounts. Females on stock diet fed vitamin A did not have more vitamin A in their adrenals than did the carotene-fed dogs in spite of the large differences in vitamin A levels elsewhere. Absorption and utilization of carotene appeared not to be affected by hypothyroidism in mature dogs but the circulating levels of vitamin A, carotene and cholesterol were significantly raised in this condition. This may be a non-specific effect on lipid levels.