Thiouracil and the conversion of carotene to vitamin A in the rat.

Several observations have indicated that the conversion of carotene to vitamin A is impaired by a hypothyroid state (1). However, the results of recent studies on the problem are not in agreement (2-6). Determinations of vitamin A in the tissues have been made in only two of the investigations (5,6). According to Wiese et al. (5), thiouracil does not affect the conversion of carotene to vitamin A as indicated by the level of vitamin A in the liver following the administration of carotene to thiouracil-fed rats on a vitamin A-low diet. Johnson and Baumann (6) found that less vitamin A is formed from carotene when the thyroid activity is impaired with thiouracil in the drinking water or with thiourea in the food. We were investigating this problem when the recent papers appeared. Because our results seemed to confirm the findings of Johnson and Baumann (6), that aspect of the study was discontinued. However, the summarized findings are reported here owing to the importance of the problem and the lack of agreement in the literature.