The Utilization of Carotene by Hypothyroid Rats

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Abstract
The conversion of carotene to vitamin A and the distribution of vitamin A was studied in young growing or mature rats made hypothyroid with thiouracil. On administration of small doses of carotene a relatively greater amount of vitamin A was recovered from the livers of the hypothyroid rats than from the normal controls. The difference disappeared with a 20-fold increase in dose. A decrease in the utilization rate of vitamin A for metabolic functions by hypothyroid rats was demonstrated when hypothyroid and normal rats were depleted of similar preexisting stores since larger amounts were found in the deficient animals than in the controls. Growth retardation through caloric restriction in normal rats likewise resulted in larger residual vitamin A stores in quantities comparable with those found in hypothyroid rats. The removal rate of a given dose of carotene in oil from the gastrointestinal tract was similar in normal and hypothyroid rats. This indicates a similarity of absorption rates in the two groups for the dose employed. Normal male rats were capable of accumulating measurable amounts of vitamin A in their kidneys following carotene supplementation. Female rats whether normal or thyroid-deficient had only insignificant accumulation of vitamin A in their kidneys. Hypothyroidism and growth retardation due to caloric restriction were factors causing the disappearance of these vitamin A deposits in the male rat. It is concluded that body weight and growth rather than basal metabolic rate govern the utilization of vitamin A and that neither absorption, transformation nor utilization of carotene is affected directly by thyroid activity.